Crystals Of NaCl Structure Research Articles

Theoretical evaluation of crystalline state properties of chalcogenide crystals with sodium chloride structure

Hellmann and Ali-Hasan forms of repulsive potential functions have been applied to compute the values of cohesive energy, atomization energy, force constant, IR absorption frequency, Debye temperature, Gruneisen parameter, Anderson-Gruneisen parameter and Moelwyn-Hughes parameter for 45 chalcogenide crystals of NaCl-structure. The results are reported here. Calculations are also performed for the estimation of first order volume dependence of Gruneisen parameter commonly known as second Gruneisen parameter using expressions of higher order derivatives of interaction potential within the frame-work of Dugdale and MacDonald theory. The high pressure behaviour of these crystals have also been studied.

Analysis of interionic potentials in chalcogenide crystals with sodium chloride structure

Values of cohesive energy, atomization energy, force constant, IR absorption frequency, Debye temperature, Gruneisen parameter, Anderson-Griineisen parameter and Moelwyn-Hughes parameter for 45 chalcogenide crystals of NaCl-structure are reported here. These parameters have been obtained by using the Born-Mayer and Varshini-Shukla interionic short-range repulsive interactions. Calculations have also been performed for the computation of first order volume dependence of Griineisen parameter commonly known as second Griineisen parameter using expressions of higher order derivatives of interaction potential within the frame work of Dugdale and MacDonald theory. The high pressure behaviour of these crystals have also been studied.

Raman scattering from epitaxial TaNx(0.94≤x≤1.37) layers grown on MgO(001)

Single-crystal group-V δ-TaNx transition-metal (TM) nitride layers, with x ranging from 0.94 to 1.37, are grown on MgO(001) and analyzed by Raman spectroscopy. In perfect NaCl-structure crystals, the first-order Raman modes are not allowed by the Oh crystal symmetry. Since first-order acoustic and optical bands are observed for all δ-TaNx samples, this indicates the presence of point defects, even for stoichiometric TaN. The large mass difference between Ta and N leads to a correspondingly large separation between the first-order acoustic and optical bands centered at 150 and 550 cm−1, respectively. A Raman peak at 440 cm−1, overlapping the first-order optical band, is identified as the difference between the first-order optical and acoustic modes. Contrary to the case for most TM nitrides, there is no peak visible in the second-order high-frequency range (700–1500 cm−1). In addition, the first-order transverse (120 cm−1) and longitudinal acoustic peaks (175 cm−1) do not exhibit the frequency shift with N/Ta ratio x (i.e., with the valence electron density) that has been reported for group-IV TM nitrides. This is consistent with the resistivity of TaNx remaining essentially constant, as well as with the very weak dependence of the lattice parameter as a function of the N/Ta ratio, and suggests that the wide single-phase region in this system is due to the presence of isoelectronic antisite substitutions rather than, as has been proposed previously, cation and anion vacancies.

Brittle-to-ductile transition and slip band structure in ionic crystals

Abstract The interrelation between the brittle-to-ductile transition (BDT) and slip structure has been examined using NaCl and KCl crystals. In pre-cracked crystals of NaCl and KCl having a 〈100〉 axis, the fracture toughness K∗ic increases above 300 and 220 K, and the BDT occurs at 510 and 430 K respectively. Scale-like wavy lines are revealed ahead of a pre-crack on etched fracture faces of NaCl tested above 300 K. As the temperature increases, the step height and density of the lines are increased but the lines are more restricted near the pre-crack front. In NaCl and KCl crystals without pre-cracking, the critical resolved shear stress for {110} main slip is very low above 77 K, while that for {010} slip markedly increases as temperature decreases below 500 and 300 K respectively. In 〈100〉-uniaxial tests for non-pre-cracked NaCl crystals, the BDT takes place at around 380 and 480 K at strain rates of 5.5 × 10−6 and 5.5 × 10−5s−1 respectively. Above the BDT temperature, wavy slip lines are abundantly produced by the marked operation of cross-slip. A mechanism based on the crack tip shielding through the cross-slip of dislocations along a crack front is proposed to explain observed crack face morphology as well as the BDT in ionic crystals of NaCl structure.

Spin-orbit coupling, spin-lattice relaxation, and spin-memory studies ofF-center-molecular-ion pairs in alkali halides

The spin-orbit parameter Δ, spin-lattice relaxation timeT1, and spin-mixing parameter ɛ ofFH(OH−) andFH(CN−) centers in several alkali halides have been studied with magnetic circular dichroism at ∼2 K. A close comparison of the experimental results before and after optically induced association of theF center with the molecular ion has been made. In crystals of NaCl structure the negative spin-orbit parameter Δ changes little betweenF andFH centers in the same host. For CsCl and CsBr two values of Δ have been derived forFH(CN−) centers with axis parallel or perpendicular to the magnetic field. In all studied systems, the spin-lattice relaxation timeT1 is already shorter before aggregation compared toF formation. The spin-mixing parameter ɛ decreases slightly forFH(OH−) compared toF centers, while it increases drastically forFH(CN−) defects and reaches its maximum possible value ɛ=0.5 in cesium halides. First attempts to interpret these magneto-optical results will be presented.

A model for stress-induced migration of tilt grain boundaries in crystals of NaCl structure

We present a qualitative model for stress-induced migration of tilt grain boundaries which we have observed during creep of sodium chloride. The elementary mechanism of the model consist in the propagation of the ledge associated to an extrinsic grain boundary dislocation. The model appears as an extension of the mechanism for growth of deformation twins and is compatible with but more general than Read's and Shockley's model for migration of low angle boundaries.

The elastic constants and their temperature and pressure derivatives of AgBr-AgCl mixed crystals

The three independent second-order elastic constants and their temperature and pressure derivatives have been measured for four AgBr-AgCl mixed crystals, with 19.5, 39.1, 56.6 and 78.7 mole % AgCl, using the ultrasonic pulse-echo technique at room temperature. The explicit temperature dependence of the elastic constants is calculated and is found to be much larger than that of other NaCl structure crystals. The violation of the Cauchy relation C 12 = C 44 is found to be significant and increases between AgBr and AgCl. The high temperature limit of the Gruneisen parameter is calculated from the elastic data. A comparison is made between the elastic properties of the silver halides and the alkali halides.

Quasiselection rule for infrared absorption by NaCl-structure crystals

A selection rule forbids splitting of a reststrahl phonon into two phonons on the same branch. We propose that interaction (summation or difference) of a reststrahl phonon with two acoustical or two optical phonons is much weaker than the interaction with one acoustical and one optical phonon. This quasiselection rule is useful in explaining several multiphonon-absorption results including the small number of two-phonon peaks observed in NaCl-structure crystals.

Piezo-optic birefringence in NaCl structure crystals. III

Following the theoretical approach of Bansigir and Iyengar, an expression for the stress-optical coefficient q44 is developed. Using experimental values of q44, ionic polarizibilities of some ions are calculated. Also, the wavelengths at which q44 = 0 are evaluated for a few crystals. Both ionic polarizibilities and reversal wavelengths are found to be in good agreement with reported values.

Some Aspects of Covalent Bonding in NaCl Structure Crystals: Application to the Lattice Dynamics of MgO

Some Aspects of Covalent Bonding in NaCl Structure Crystals: Application to the Lattice Dynamics of MgO