Hydrostatic Pressure Derivatives Research Articles

Uniaxial and hydrostatic pressure effects in α-RuCl3 single crystals via thermal-expansion measurements

We present high-resolution thermal-expansion and specific-heat measurements of single crystalline α-RuCl3. An extremely hysteretic structural transition expanding over 100 K is observed by thermal-expansion along both crystallographic axes, which we attribute to a change of stacking sequence of the RuCl3 layers. Three magnetic transitions are observed, which we link to the different stacking sequences. Using our data and thermodynamic relations, we derive the uniaxial and hydrostatic pressure derivatives of all three magnetic transitions. Our results demonstrate that magnetic order of the major transition at ∼7 K should be totally suppressed by very moderate pressures of 0.3 GPa. Finally, we discuss why our results differ from recent hydrostatic pressure measurements and suggest a possible route to reaching the spin-liquid state in α-RuCl3.

An increase in Tc under hydrostatic pressure in the superconducting doped topological insulator Nb0.25Bi2Se3

We report a positive hydrostatic pressure derivative of the superconducting transition temperature in the doped topological insulator Nb0.25Bi2Se3 via dc SQUID magnetometry in pressures up to 0.6 GPa. This result is contrary to reports on the homologues CuxBi2Se3 and SrxBi2Se3 where smooth suppression of Tc is observed. This difference may be attributable to an electronic structure composed of multiple bands whereas the other materials in the superconducting doped Bi2Se3 family are believed to be single-band.

Enhancement of the Critical Temperature ofHgBa2CuO4+δby Applying Uniaxial and Hydrostatic Pressure: Implications for a Universal Trend in Cuprate Superconductors

It is well known that the superconducting transition temperature (T(c)) of cuprate superconductors can be enhanced by varying certain structural and electronic parameters, such as the flatness of the CuO₂ planes or their doping level. We determine the uniaxial and hydrostatic pressure derivatives of T(c) in the structurally simple tetragonal compound HgBa₂CuO(4+δ) near optimal doping. Our results provide experimental evidence for two further methods to enhance T(c): (i) reducing the area of the CuO₂ planes, and (ii) increasing the separation of the CuO₂ planar groups. T(c) is found to couple much more strongly to the ratio c/a of the lattice constants than to the unit cell volume. A comparison with prior results for structurally more complicated cuprates reveals a general trend of uniaxial pressure derivatives with T(c).

Ultrasonic study of the temperature and pressure dependences of the elastic properties of ceramic dimolybdenum carbide ( -Mo2C)

Pulse-echo overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic samples of dimolybdenum carbide (α-Mo2C) as functions of temperature in the range 130–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. The temperature dependences of the shear elastic stiffness (μ) and Young's modulus (E) show normal behaviour and can be approximated by a conventional model for vibrational anharmonicity. The longitudinal elastic stiffness (CL) increases with decreasing temperature and shows a knee at about 200 K; the decrease in slope below the knee indicates longitudinal acoustic-mode softening. The adiabatic bulk modulus (BS) is also affected by the mode softening below 200 K. The values obtained for the acoustic Debye temperature (ΘD) for ceramic α-Mo2C agree well with the thermal Debye temperature determined previously from heat capacity measurements. The velocities of both the longitudinal and shear ultrasonic waves in ceramic α-Mo2C increase approximately linearly with pressure: both the long-wavelength longitudinal and shear acoustic modes stiffen under pressure. The values determined at room temperature for the hydrostatic-pressure derivative (∂μ/∂P)P=0 of the shear stiffness is similar to those found for ceramic TiC and TaC; while (∂CL/∂P)P=0 and (∂BS/∂P)P=0 have large values, possibly due to the defect microstructure of ceramic α-Mo2C.

Ultrasonic study of the temperature and hydrostatic-pressure dependences of the elastic properties of polycrystalline cementite (Fe3C)

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the dependences of the elastic stiffness moduli of polycrystalline cementite (Fe3C) on temperature in the range 75–295 K and hydrostatic pressure up to 0.1 GPa at room temperature. The longitudinal stiffness (CL) and adiabatic bulk modulus (BS) stiffen, while the shear stiffness (μ) and Young's modulus (E) soften with decreasing temperature. The ultrasonic velocities increase approximately linearly with pressure, much more steeply for the longitudinal than the shear mode. The values obtained at 295 K for the hydrostatic-pressure derivatives (∂CL/∂P)P=0, (∂μ/∂P)P=0 and (∂BS/∂P)P=0 of cementite are 7.9 ± 1.7, 1.4 ± 0.1 and 6.1 ± 1.7, respectively: the zone-centre acoustic phonons stiffen under pressure. The longitudinal (γL), shear (γS) and mean (γel) acoustic-mode Grüneisen parameters of cementite are positive; γS is markedly smaller than γL indicating that the shear acoustic modes are less anharmonic than the longitudinal modes.

Ultrasonic determination of the elastic and nonlinear acoustic properties of transition-metal carbide ceramics: TiC and TaC

Pulse-echo overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic transition-metal carbides TiC and TaC as functions of temperature in the range 135–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. The carbon concentration of each ceramic has been determined using an oxidation method: the carbon-to-metal atomic ratios are both 0.98. In general, the values determined for the adiabatic bulk modulus (B S), shear stiffness (μ), Young's modulus (E), Poisson's ratio (σ) and acoustic Debye temperature (ΘD) for the TiC and TaC ceramics agree well with the experimental values determined previously. The temperature dependences of the longitudinal stiffness (C L) and shear stiffness measured for both ceramics show normal behaviour and can be approximated by a conventional model for vibrational anharmonicity. Both the bulk and Young's moduli of the ceramics increase with decreasing temperature and do not show any unusual effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic pressure derivatives of elastic stiffnesses and the acoustic-mode Gruneisen parameters. The values determined at 295 K for the hydrostatic pressure derivatives (∂C L/∂P)P = 0, (∂μ/∂P)P = 0 and (∂B S/∂P)P = 0 for TiC and TaC ceramics are positive and typical for a stiff solid. The adiabatic bulk modulus B S and its hydrostatic pressure derivative (∂B S/∂P)P = 0 of TiC are in good agreement with the results of recent high pressure X-ray diffraction measurements and theoretical calculations. The longitudinal (γL), shear (γS) and mean (γel) acoustic-mode Gruneisen parameters of TiC and TaC ceramics are positive: the zone-centre acoustic phonons stiffen under pressure. The shear γS is much smaller than the longitudinal γL. The relatively larger values estimated for the thermal Gruneisen parameter γth in comparison to γel for the TiC and TaC ceramics indicate that the optical phonons have larger Gruneisen parameters. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic TiC and TaC.

Temperature and pressure dependences of the elastic properties of ceramic boron carbide (B4C)

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic boron carbide (B4C) as functions of temperature in the range 160–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. B4C is an elastically stiff but extremely light ceramic: at 295 K, the longitudinal stiffness (CL), shear stiffness (μ), adiabatic bulk modulus (B S ), Young's modulus (E) and Poisson's ratio (σ) are 498 GPa, 193 GPa, 241 GPa, 457 GPa and 0.184, respectively. In general, the adiabatic bulk modulus B S agrees well with both experimental and theoretical values determined previously and is approximately constant over the measured temperature range. Both E and μ increase with decreasing temperature and do not show any unusual effects. The values determined at 295 K for the hydrostatic-pressure derivatives (∂ CL/∂P)P=O, (∂μ/∂P)P=O and (∂B S /∂P)P=O are 5.7 ± 0.3, 0.78 ± 0.4 and 4.67 ± 0.3, respectively. The hydrostatic-pressure derivative (∂B S /∂P)P=O of the bulk modulus is found to be comparable with that estimated previously from dynamic yield strength measurements. The effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Gru neisen parameters. The longitudinal (γL), shear (γS), and mean (γel) acoustic-mode Gruneisen parameters of B4C are positive: the zone-centre acoustic phonons stiffen under pressure in the usual way. Knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic B4C. Since the acoustic Debye temperature ΘD (=1480 K) is very high, the shear modes provide a substantial contribution to the acoustic phonon population at room temperature.

Elastic and non-linear acoustic properties and thermal expansion of cerium metaphosphate glasses

To test predictions of the soft potential model (SPM) for the thermal and acoustic properties of glasses, the thermal expansion and the ultrasonic wave velocity and attenuation have been measured in cerium metaphosphate glasses with compositions in the vicinity of (Ce 2O 3) 0.25(P 2O 5) 0.75. The ultrasonic wave velocities have been determined as functions of temperature and hydrostatic pressure; the results provide the temperature dependences of the adiabatic elastic stiffnesses C 11 and C 44 and related elastic properties, and the hydrostatic-pressure derivatives (∂ C 11/∂ P) P=0 and (∂ C 44/∂ P) P=0 of the elastic stiffnesses and (∂ B S/∂ P) P=0 of the bulk modulus. The longitudinal ultrasonic wave velocities increase under pressure. The hydrostatic pressure derivative (∂ B S/∂ P) P=0 of the bulk modulus B S is positive: when compressed, the cerium metaphosphate glasses show a normal volume elastic response. However, the pressure derivative (∂ C 44/∂ P) P=0 of the shear modulus is negative but small, indicating weak softening of shear modes under pressure. The shear wave ultrasonic attenuation is characterised by a broad peak; the calculated relaxation parameters are consistent with phonon-assisted relaxation of two-level systems. The results found for C IJ and (∂ C IJ /∂ P) P=0 are used to determine the long-wavelength acoustic-mode Grüneisen parameters, which quantify the vibrational anharmonicity and are needed to obtain the acoustic mode contribution to thermal expansion. The temperature dependence of the shear wave ultrasound velocity, after subtraction of the relaxation and anharmonic contributions, follows a linear law as predicted by the SPM for relaxation of soft harmonic oscillators. At low temperatures the excess low-energy vibrational states provide a negative contribution to thermal expansion, which can be understood on the basis of the SPM.

Ultrasonic study of the elastic and nonlinear acoustic properties of ceramic aluminum nitride

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of aluminum nitride (AlN) ceramic samples as functions of temperature in the range 100–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. Aluminum nitride is an elastically stiff but light ceramic: at 295 K, the longitudinal stiffness (C L), shear stiffness (μ), adiabatic bulk modulus (B S), Young's modulus (E) and Poisson's ratio (σ) are 373 GPa, 130 GPa, 200 GPa, 320 GPa and 0.234, respectively. The temperature dependences of C L and B S show normal behaviour and can be approximated by the conventional model for vibrational anharmonicity. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Gruneisen parameters. The values determined at 295 K for the hydrostatic-pressure derivatives (∂C L/∂P) P=0, (∂μ/∂P) P=0 and (∂B S/∂P) P=0 are 4.7 ± 0.1, 0.22 ± 0.03 and 4.4 ± 0.15, respectively. The adiabatic bulk modulus B S and its hydrostatic-pressure derivative (∂B S/∂P) P=0 are in good agreement with the results of recent high pressure X-ray diffraction measurements and theoretical calculations. The longitudinal (γL), shear (γS), and mean (γel) acoustic-mode Gruneisen parameters of AlN are positive: the zone-centre acoustic phonons stiffen under pressure. The shear γS (=0.006) is much smaller than the longitudinal γL (=1.09) accounting for the low thermal Gr¨neisen parameter γth (=0.65) obtained for this ceramic: since the acoustic Debye temperature ΘD (=980 ± 5 K) is so high, the shear modes play an important role in acoustic phonon population at room temperature. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic AlN.

Ultrasonic determination of the temperature and hydrostatic pressure dependences of the elastic properties of ceramic titanium diboride

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of titanium diboride (TiB2) ceramic samples as functions of temperature in the range 130–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. TiB2 is an elastically stiff but light ceramic: at 295 K, the longitudinal stiffness (CL), shear stiffness (μ), adiabatic bulk modulus (BS), Young's modulus (E) and Poisson's ratio (σ) are 612 GPa, 252 GPa, 276 GPa, 579 GPa and 0.151, respectively. The adiabatic bulk modulus BS is in good agreement with the results of recent theoretical calculations. All elastic moduli increase with decreasing temperature and do not show any pronounced unusual effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Gruneisen parameters. The values determined at 295 K for the hydrostatic-pressure derivatives (∂CL/∂P)P=0, (∂μ/∂P)P=0 and (∂BS/∂P)P=0 are 7.29 ± 0.1, 2.53 ± 0.1 and 3.91 ± 0.1, respectively. The hydrostatic-pressure derivative (∂BS/∂P)P=0 of the bulk modulus of TiB2 ceramic is found to be larger than that estimated previously from uniaxial shock-wave loading experiments. The longitudinal (γL), shear (γS), and mean (γel) acoustic-mode Gruneisen parameters of TiB2 are positive: the zone-centre acoustic phonons stiffen under pressure in the usual way. Since the acoustic Debye temperature ΘD (=1190 K) is very high, the shear modes provide a substantial contribution to the acoustic phonon population at room temperature. Knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic TiB2.

Ultrasonic study of the temperature and pressure dependences of the elastic properties of β-silicon nitride ceramic

Ultrasonic wave velocity measurements have been used to determine the elastic stiffness moduli and related elastic properties of high-purity, dense β-Si3N4 ceramic samples as functions of temperature in the range 150–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. Due to its covalently bonded, rigid structural framework β-Si3N4 is an elastically stiff material; the elastic stiffness moduli of the ceramic at 295 K are: CL = 396 GPa, μ = 119 GPa, BS = 238 GPa, E = 306 GPa, Poisson's ratio σ = 0.285. The longitudinal elastic stiffness CL increases with decreasing temperature and shows a knee at about 235 K; the decrease in slope below the knee indicates mode softening. The shear elastic stiffness μ shows mode softening which results in a plateau centred at about 235 K and an anomalous decrease with further reduction in temperature. The hydrostatic-pressure derivatives of elastic stiffnesses at 295 K are (∂CL/∂P)P=0 = 4.5 ± 0.1, (∂BS/∂P)P=0 = 4.3 ± 0.1 and (∂μ/∂P)P=0 = 0.17 ± 0.02 (pressure < 0.12 GPa). An interesting feature of the nonlinear acoustic behaviour of this ceramic is that, in the pressure range above 0.12 GPa, the values obtained for (∂μ/∂P)P=0 and the shear mode Gruneisen parameter (γS) are small and negative, indicating acoustic-mode softening under these higher pressures. Both the anomalous temperature and pressure dependences of the shear elastic stiffness indicate incipient lattice shear instability. The shear γS(=0.005) is much smaller than the longitudinal γL(=1.18) accounting for the thermal Gruneisen parameter γth(=1.09): since the acoustic Debye temperature ΘD(=923 ± 5 K) is so high, the shear modes play an important role in acoustic phonon population at room temperature. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic β-Si3N4.

Ultrasonic study of the elastic and nonlinear acoustic properties of single-crystal triglycine sulphate in the ferroelectric and paraelectric phases

Abstract An experimental ultrasonic study has been made of the elastic and nonlinear acoustic behaviour of single-crystal triglycine sulphate (TGS) in both the ferroelectric and paraelectric phases. The ultrasonic wave velocities for 12 modes of vibration propagated along the [100], [010], [001] and [101] directions have been measured as functions of temperature in the range 293-338 K, which covers the Curie temperature T C, and hydrostatic pressure up to 0.2 GPa at selected temperatures in the same temperature range. The results provide ten out of the 13 elastic stiffness tensor components C ij of TGS (the exceptions being C 12, C 23 and C 25) and their hydrostatic-pressure derivatives as a function of temperature. Marked anomalies are observed in the C ij (T) in the vicinity of T c, with that for C 33 being the most pronounced. The elastic stiffness tensor component C 22, which determines the temperature dependence of the velocity of longitudinal ultrasonic waves propagated along the ferroelectric axis,...

Acoustic Vibrational Properties and Fractal Bond Connectivity of Praseodymium Doped Glasses

The velocities of longitudinal and shear ultrasonic waves propagated in the (Pr2O3)x(P2O5)1-x glass system, where x is the mole fraction of Pr2O3 and (1 - x) is the mole fraction of P2O5, have been measured as functions of temperature and hydrostatic pressure. The temperature dependencies of the second order elastic stiffness tensor components (SOEC) CS IJ , which have been determined from the velocitydata between 10 and 300 K, show no evidence of phonon mode softening throughout the whole temperature range. The elastic stiffnesses increased monotonically, the usual behaviour associated with the effect of the phonon anharmonicityof atomic vibration. At low temperatures, strong phonon interactions with two-level systems have been observed. The ultrasonic wave attenuation of longitudinal and shear waves is dominated bya broad acoustic loss peak whose height and peak position are frequencydependent. This behaviour is consistent with the presence of thermally activated structural relaxation of the two-level systems in these glasses. The fractal bond connectivity of these glasses, obtained from the elastic stiffnesses determined from ultrasonic wave velocities, has a value between 2.32 to 2.55, indicating that their connectivitytends towards having a threedimensional character. The hydrostatic pressure dependencies of longitudinal ultrasonic waves show a slight increase with pressure. As a consequence, the hydrostatic pressure derivatives ( CS11/ P)P=0 of the elastic stiffness CS11/ and (BS/P)P=0 of the bulk modulus BS of (Pr2O3)x(P2O5)1-x glasses are positive. The bulk modulus increases with pressure, and thus these glasses stiffen under pressure, which is associated with the normal elastic behaviour. The GrÜneisen parameter approach has been used to quantifythe vibrational anharmonicityof the long-wavelength acoustic phonons in these glasses.

Non-linear acoustic properties and acoustic-mode vibrational anharmonicity of 18R martensite Cu-Zn-Al shape-memory alloy

To shed further light on the dynamics of the martensitic transformation in shape-memory alloys, measurements have been made of the effects of hydrostatic pressure on the velocities of 15 different pure ultrasonic modes propagated at room temperature in a single-crystal alloy in its 18R martensitic phase. Although the point symmetry group of the 18R martensite is monoclinic, the elastic behaviour can be reasonably described in terms of an orthorhombic structure; the further monoclinicity beyond such an orthorhombic representation is very small and can be neglected to a first approximation. The results obtained for the nine independent second-order elastic stiffness tensor components in the 18R structure and their hydrostatic-pressure derivatives have been used to calculate the mode Grüneisen parameters , which quantify the vibrational anharmonicity of long-wavelength acoustic phonons. Results are compared with those computed from TOEC data measured on a -phase Cu-Zn-Al single crystal with a similar composition. At room temperature, about 28 K below the martensitic transition temperature (= 323 K), the longitudinal mode have normal, positive values; however, those associated with some shear modes have negative values. A pronounced minimum has been found in the Grüneisen parameter of the shear mode propagated along the [111] direction with polarization vector. It is concluded that the lattice stability against this particular shear mode decreases as the sample approaches the martensitic transition.

Ultrasonic study of the temperature and pressure dependences of the elastic properties of a single-crystal Heusler structure Cu41Mn20Al39 alloy

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic-stiffness tensor components Cu and the adiabatic bulk modulus, BS, of a ferromagnetic Heusler structure Cu41Mn20Al39 at % alloy single crystal as functions of temperature in the range 14–300 K and hydrostatic pressure up to 0.2 GPa at room temperature. At 295 K the elastic stiffnesses are: C11=133 GPa, C44=92 GPa, C′ (=(C11−C12)/2)=17 GPa, C12=99 GPa, CL(=C11+C44−C′)=205 GPa, and BS (=C11−4C′/3)=106 GPa. Cu41Mn20Al39 is a comparatively soft material elastically because its elastic properties are influenced strongly by magnetoelastic effects. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to obtain the hydrostatic-pressure derivatives of the elastic – stiffness tensor components. At 295 K (∂C11/∂P)P=0, (∂C44/∂P)P=0, (∂C′/∂P)P=0, (∂C12/∂P)P=0, (∂CL/∂P)P=0, and (∂BS/∂P)P=0 are 5.0±0.1, 3.0±0.1, 1.0±0.2, 3.0±0.3, 7.7±0.4 and 3.7±0.4, respectively. Application of hydrostatic pressure does not induce acoustic-mode softening: the pressure derivatives (∂CIJ/∂P)P=0 and (∂BS/∂P)P=0 and the acoustic-mode Gruneisen parameters are positive. An interesting feature of the non-linear acoustic behaviour of this alloy is that the value obtained for (∂C′/∂P)P=0, associated with the softer shear mode propagated along the [1 1 0] direction and polarized along the [1 1 0] direction, is small in comparison with those of the other shear and longitudinal modes. The Gruneisen parameter of this mode, and hence its vibrational anharmonicity, is much larger than those of the other long-wavelength acoustic phonon modes. © 1998 Kluwer Academic Publishers

Elastic and nonlinear acoustic properties and thermal expansion of rare-earth metaphosphate glasses

Abstract The thermal expansions and the ultrasonic wave velocities and attenuations of (Sm2O3)0.234(P2O5)0.776 and (La2O3)0.222 (P2O5)0.778 and mixed (La2O3)x (Sm2O3)y(P2O5)0.75 (where x + y = 0.25) metaphosphate glasses have been measured as functions of temperature. The change in the ultrasonic wave velocity induced by application of hydrostatic pressure up to 0.16 GPa has also been measured at selected temperatures between room temperature and 375 K. The experimental results provide the temperature dependences of the adiabatic elastic stiffnesses C11 and C44 and related elastic properties and provide the hydrostatic-pressure derivatives (∂C 11/∂P)P=0 and (∂C 44/∂P)p=0 of the elastic stiffnesses and (∂B S/∂P)P=0 of the bulk modulus. The results obtained for C IJ and (∂C IJ/∂P)P=0 are used to determine the long-wavelength acoustic-mode Grüneisen parameters; these quantify the vibrational anharmonicity, which is essential information for developing the acoustic mode contribution to the thermal expansion of the glasses. Hence the ultrasonic velocity measurements in metaphosphate glasses modified with Sm3+ and La3+ ions enable separation of these acoustic-mode contributions from those due to the excess modes. Using the soft-potential model, it has been shown that, at low temperatures, the excess low-energy vibrational states provide negative contributions to the thermal expansion and to the nonlinear acoustic properties. For the mixed metaphosphate glasses of the type (La2O3)x(Sm2O3)y(P2O5)0.75, the acoustic-mode softening induced by Sin3+ is negated by the addition of La3+; as a result the acoustic-mode Grüneisen parameters and the thermal expansion are extremely small.

Ultrasonic study of the elastic and nonlinear acoustic properties of the paramagnetic Cr-5 at.% V single-crystal alloy

Abstract To assist understanding the effects of pressure on the magnetoelastic interactions in antiferromagnetic Cr alloys, the elastic and nonlinear acoustic properties of single-crystal Cr-5 at. %V alloy, which is paramagnetic at all temperatures, have been studied by the ultrasonic pulse-echo-overlap technique. Measurements of the hydrostatic-pressure dependences of the velocities of ultrasonic modes, propagated along the [100] and [110] directions, have been used to obtain the hydrostatic-pressure derivatives (∂CIJ∂P)P=0 of the elastic-stiffness tensor components CIJ . At 296 K, (∂CIJ/∂P}P=0 , (∂CL/∂P)P=0 , (∂C44/∂P)P=0 , (∂C1/∂P)P=0 and (∂B2/∂P)P=0 are 9·1 ± 0·1, 8·7 ± 0·1, 1·0 ± 0.02, 1·74 ± 0·02 and 6·8 ± 0·1 respectively. To establish the vibrational anharmonicity of the long-wavelength acoustic modes, the results obtained for CIJ and (∂CIJ/∂P)P=0 have been used to calculate the corresponding Gruneisen parameters. The mean long-wavelength acoustic Gruneisen parameter γd(= 1·43) is similar to the t...

Elastic and anelastic properties, vibrational anharmonicity, and fractal bond connectivity of superionic glasses.

To quantify the thermally activated relaxations of the mobile silver ions in superionic silver phosphosulphate ~Ag2SO4!x~AgPO3!(12x) and phosphosulphide ~Ag2S!x~AgPO3!(12x) glasses, the broad attenuation peaks reported previously have been analyzed in terms of a Gaussian-type energy distribution. The parameters obtained are used to determine the influence of thermally activated relaxation processes on the temperature dependence of the ultrasonic wave velocity measured between 1.5 and 300 K. After subtraction of the relaxation effects together with those due to anharmonic interactions, another contribution to the temperature dependence of the ultrasonic velocity remains below 100 K, which follows a linear temperature dependence—as predicted by the soft-potential model ~SPM! for relaxation of soft harmonic oscillators. The soft HO relaxation contribution to the ultrasonic velocity temperature dependences of silver phosphate-based glasses has a similar magnitude to those determined previously for lanthanide metaphosphate glasses. While the silver phosphate-based glasses have skeletons that are comprised of long chains of phosphate ions, the lanthanide metaphosphate glasses are close to having a three-dimensional structure. The agreement of the excess contribution to the temperature dependence of the ultrasonic velocity with the predictions of the SPM for both types of glass, in spite of their complete differences in structure, is further evidence for the universal applicability of the soft-potential model. To determine the vibrational anharmonicity of the long wavelength acoustic modes in superionic glasses, the hydrostatic pressure derivatives of the second-order elastic stiffness tensor components have been measured for these ~Ag2SO4!x~AgPO3!(12x), ~Ag2S!x~AgPO3!(12x), vitreous AgPO3 and also for silver iodide molybdate ~AgI!0.75~Ag2MoO4!0.25. To examine further the effects of vibrational anharmonicity, the thermal expansions of the vitreous phosphates have also been measured. The linear thermal expansion coefficient becomes anomalously negative at lower temperatures for the ~Ag2SO4!x~AgPO3!(12x) and ~Ag2S!x~AgPO3!(12x) glasses. The wide variations found between the elastic and nonlinear acoustic properties of superionic silver phosphate, molybdate, and borate glasses stem from differences in the bonding and connectivities of the glass skeletons.

Measurement of the In0.52Al0.48As valence-band hydrostatic deformation potential and the hydrostatic-pressure dependence of the In0.52Al0.48As/InP valence-band offset.

We have measured the ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As valence-band hydrostatic deformation potential from the hydrostatic-pressure dependence of the ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As/InP valence-band offset which was measured from 0 to 35 kbar at room temperature. Due to the type-II band lineup, the radiative recombinations across the InP band gap and between the InP conduction band and the ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As valence band were both observed in the photoluminescence spectra. This enables us to measure directly the changes of the valence-band offset under pressure. The hydrostatic-pressure derivative of the valence-band offset was measured to be 0.0\ifmmode\pm\else\textpm\fi{}0.4 meV/kbar. The predictions of the pressure dependence from band-offset models (dielectric midgap and model-solid theories) agree with the measurement to within 1 meV/kbar. The ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As valence-band hydrostatic deformation potential is found to be -0.8 eV which compares well with the dielectric midgap theory. Using the reported pressure dependence of the GaAs/AlAs valence-band offset, the valence-band hydrostatic deformation potentials of ${\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ ($0\ensuremath{\le}x\ensuremath{\le}0.52$) are linearly interpolated as $\ensuremath{-}1.9x+0.2$ eV.

Anisotropy of the elastic and nonlinear acoustic properties of dense textured Bi2Sr2CaCu2O8+y.

A description of the anisotropy of elastic and nonlinear acoustic properties of dense (95% of theoretical), highly textured, sinter-forged, ceramic ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$ (Bi-2:2:1:2) high-${\mathit{T}}_{\mathit{c}}$ superconductors is provided from measurements of the velocities of longitudinal and shear ultrasonic waves as functions of temperature and hydrostatic pressure. A high proportion of the grains are aligned preferentially with the c axis along the forging direction in this material, while it is essentially isotropic in the plane normal to the forging axis. A complete set of the elastic stiffness tensor components has been determined as a function of temperature from 10 to 290 K, treating this material as if it had cylindrical symmetry. The results are used to compute the orientation dependences of the Young's modulus and Poisson's ratio which are compared with those estimated for single crystal Bi-2:2:1:2. The main finding is a marked anisotropy of the elastic stiffness moduli and their dependences on pressure. The elastic and nonlinear acoustic properties of the textured and single-crystal Bi-2:2:1:2 comply with the behavior expected for a layerlike material with rather weak interlayer binding forces: ${\mathit{C}}_{11}$g${\mathit{C}}_{33}$, the linear compressibility and thermal expansion along to the c axis, i.e., normal to the layers, being substantially larger than in the ab plane. The hydrostatic pressure derivative (\ensuremath{\partial}${\mathit{C}}_{33}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$(=21.3) of the elastic stiffness tensor component, which corresponds to a longitudinal wave propagated along the c axis, is larger than that (\ensuremath{\partial}${\mathit{C}}_{11}$/\ensuremath{\partial}P${)}_{\mathit{P}=0}$(=15.1), for such a wave propagated in the ab plane. The anisotropies of the linear compression and the acoustic mode Gr\uneisen parameters are also typical of a layerlike material.