Temperature Dependent Raman Spectroscopic Studies Research Articles

Phase transition, crystal water and low thermal expansion behavior of Al2−2x(ZrMg)xW3O12·n(H2O)

Al2−2x(ZrMg)xW3O12 for 0≤x≤1.0 are synthesized to reduce the phase transition temperature of Al2W3O12. It is found that the incorporation of (ZrMg)6+ into the lattice of Al2W3O12 not only reduces its orthorhombic-to-monoclinic phase transition temperature but also elevates its softening temperature, broadening its applicable temperature range considerably. Al2−2x(ZrMg)xW3O12 with x<0.5 exhibit low coefficients of thermal expansion (CTEs) and non-hygroscopicity, while those for x≥0.7 are obviously hygroscopic and the CETs decrease with increasing the content of (ZrMg)6+ so that Al0.2(ZrMg)0.9W3O12 and ZrMgW3O12 exhibit negative thermal expansion. Temperature-dependent Raman spectroscopic study shows the hardening of W–O bonds above 373K which is attributed to the release of crystal water. The effect of crystal water on the thermal expansion property is discussed based on the hydrogen bond between H in crystal water and electronegative O in Al(ZrMg)–O–W linkages.

Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3

Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO3 over a temperature range of 20–300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a Néel transition at TN ∼ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below TN. Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near TN. An additional phonon anomaly was identified at T* ∼ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman frequencies between TN and T* suggest competing WFM and AFM interactions. A close match between the phonon frequency of B3g (3)-octahedral rotation mode with the square of sublattice magnetization between TN and T* is indicative of the presence of spin-phonon coupling in multiferroic YCrO3.

Effect of temperature on thermal expansion and anharmonicity in Cu2ZnSnS4 thin films grown by co-sputtering and sulfurization

Copper zinc tin sulfide (CZTS, made from the earth abundant and non-toxic materials) is a quaternary semiconducting compound which has received increasing interest for solar cells applications. In this study, CZTS thin film has been grown by co-sputtering Cu, Zn and Sn metal targets and sulfurizing it in H2S. XRD, SEM, EDS, XRF and optical studies show that these films are suitable for solar cell applications. The temperature-dependent Raman spectroscopic study on CZTS thin film was carried out in the temperature range of 80–450 K. A decrease in the intensity of “A” mode Raman peaks, shift of Raman peaks towards lower frequency and increase in the line width have been observed with increase in temperature. Raman “A” mode shifts from 337 cm−1 at 80 K to 329 cm−1 at 450 K. The peak at 288 cm−1 disappears when the measurement was taken at 450 K. Based on experimental results and fittings of anharmonic equation; it has been shown that these effects are due to thermal expansion and interaction of the phonon with other phonons which arise at higher temperature. These phonons interact resulting in damping of the “A” mode phonon intensity. It was concluded that in order to truly analyze multicomponent compounds, Raman studies should be carried out at low temperature.

Temperature-dependent Raman spectroscopic study on orthophosphates Ba3(PO4)2 and Sr3(PO4)2

Temperature-dependent Raman spectroscopic study on orthophosphates Ba3(PO4)2 and Sr3(PO4)2 were carried out up to 900 ℃. The change of Raman lines and crystal structure has been investigated at high-temperatures. It is found that all the Raman lines exhibit a decrease in frequency shifts and the width of the Raman lines increases with the increase in temperature. The P-O bond lengths in the crystal increase in high- temperature, but the O-P-O bond angles have less change. No phase changes have been observed under 900 ℃.

Liquid phase dynamics of molten M2S2O7 (M = K, Cs): A temperature dependent Raman spectroscopic study

The dynamics of M2S2O7 (M=K, Cs) pyrosulfate salts in the liquid state is investigated by steady-state Raman spectroscopic experiments performed at temperatures up to 600°C. The symmetric stretching modes of the S2O72− ions have been used as probes of the dynamics of these melts. Contrary to the most previous picosecond dynamics studies performed by means of Raman line profile analysis, we have employed in this work an approach that enables the extraction of valuable information concerning short-time dynamics by calculating time correlation functions of vibrational relaxation by fits in the frequency domain. The fitting method used enables the modeling of the real line profiles in a manner that is intermediate between Lorentzian and Gaussian by means of a function, which has an analytical counterpart in the time domain. The vibrational time correlation functions for both molten salts studied are rather adequately interpreted within the assumption of exponential modulation function concerning the environmental modulation in the context of Kubo–Rothschild approach and indicate that the system experiences an intermediate dynamical regime that gets only slower with increasing temperature. Continuous temperature dependence of the dephasing parameters is observed, while the temperature dependence of the dispersion parameter α indicates deviation from the simple liquid model and offers a complete picture of the way a complex liquid attains the condition of a simple one. The evolution of the dispersion parameter is indicative of the reduction of the coherence decay in the perturbation potential as a consequence of local short-lived aggregates. The experimental results are discussed in terms of theoretical models providing insight in the intermolecular coupling mechanisms.

Phase transition and thermal expansion properties of ZrV2-xPxO7

ZrV2-xPxO7 (x = 0, 0.4, 0.8, 1.0) solid solutions are prepared using a solid state reaction method. Powder X-ray diffraction (XRD) analysis reveals that the as-prepared solid solutions are of single-phase cubic type in crystal structure. Temperature dependent Raman spectroscopic studies demonstrate that the phase transition temperature of ZrV2-xPxO7 decreases with the increase of the content of P. ZrV2O7, ZrV1.6P0.4O7, ZrV1.2P0.8O7 and ZrVPO7 transform from a 3× 3× 3 superstructure to a 1× 1× 1 normal structure at about 373 K, 363 K 273 K and 213 K, respectively. The results of thermal expansion testing indicate that the temperature of positive thermal expansion changes to negative thermal expansion with the increase of P content. The temperatures of positive-to-negative thermal expansion of ZrV2-xPxO7 are 429 K, 403 K, 372 K, 390 K, 398 K and 435 K for x=0, 0.2, 0.4, 0.6, 0.8 and 1, respectively. Two phase change transformations are demonstrated in ZrV2-xPxO7 materials in our work, which is beneficial for preparing the negative thermal expansion materials at room temperature using ZrV2O7.

Temperature‐dependent Raman spectroscopy of anilinium chloride near phase transition

AbstractThe existence of various solid phases of organic ammonium halides, due to the presence of hydrogen bonding, has long been established but is limited to alkyl ammonium halides. On the other hand, the presence of such phase transitions was eluded in case of anilinium chloride. In the present work, we have undertaken a systematic temperature dependent Raman spectroscopic study on anilinium chloride, in the range 300–100–350 K. Spectral variation in the low frequency (100–400 cm−1) and high frequency (2500–3700 cm−1) regions around 350 and 260 K shows the signature of the phase transitions in this compound. These phase transitions were reversible in nature. The latter transition is in agreement with DSC measurements.magnified image

Superionic Phase Transition in KHSO4: A Temperature-Dependent Raman Investigation

Temperature-dependent Raman spectroscopic studies have been carried out on KHSO4 single crystals in the temperature range 298-493 K. A structural phase transition driven by the lattice and molecular disorder is observed at 473 K. The spectral data enable an understanding of the nature of the lattice disorder across the phase transition leading to the superionic phase. The disorder in the HSO4- polymeric hydrogen-bonded chain leading to a higher symmetry in the high temperature phase is clearly captured from our Raman results. The internal S-OH and S−O stretching modes and, to a limited extent, the external modes throw light on the disorder mechanism and the enhancement of conductivity after transition.

Raman signatures of charge ordering inK0.3WO3

Received 5 August 2009; revised manuscript received 16 November 2009; published 28 January 2010 We present polarization- and temperature-dependent Raman spectroscopic study of hexagonal tungsten bronze, K0.3WO3. The observed asymmetry in phonon line shapes indicate the presence of strong lattice anharmonicity arising due to the nonstoichiometry of the material. We observed a broad multipeak Raman feature at low frequency due to the local modes of K atoms known as local structural excitations. The observed vibrational features indicate a second-order phase transition around T=200 K accompanied by a frequency softening of low-frequency phonon modes. The observed phonon anomalies hint a physical picture involving a continuous symmetry change toward a charge-ordered state below 200 K. These observations indicate that K0.3WO3 may exhibit a weak charge-density-wave ground state at low temperatures.

High-Temperature Phase Transition Studies in a Novel Fast Ion Conductor, Na2Cd(SO4)2, Probed by Raman Spectroscopy

Temperature-dependent Raman spectroscopic studies were carried out on Na(2)Cd(SO(4))(2) from room temperature to 600 degrees C. We observe two transitions at around 280 and 565 degrees C. These transitions are driven by the change in the SO(4) ion. On the basis of these studies, one can explain the changes in the conductivity data observed around 280 and 565 degrees C. At 280 degrees C, spontaneous tilting of the SO(4) ion leads to restriction of Na(+) mobility. Above 565 degrees C, the SO(4) ion starts to rotate freely, leading to increased mobility of Na(+) ion in the channel.

A low-frequency Raman study of glassy, supercooled and molten silica and the preservation of the Boson peak in the equilibrium liquid state

A temperature dependent Raman spectroscopic study of the Boson peak in silica is being presented. For the first time experimental data are obtained not only above the glass transition temperature but also above the melting point. The intriguing temperature dependencies exhibited by the Boson peak frequency and intensity are examined in detail. A comparison of the above features between the frequency-reduced and the true excess density of vibrational states revealed that the spectrum of the latter does not exhibit the anomalous trend of the Boson peak as regards the temperature dependence of its frequency, and further the number of the excess modes decreases with increasing temperature. The observability of the Boson peak in the normal liquid state as a well-resolved peak is also discussed in the context of recent theoretical and simulation works that relate the Boson peak with the details of the potential energy landscape of the supercooled liquid.

Spectroscopic characterization of pure and cation-stabilized sodium phosphate

A systematic study was conducted of pure Na3PO4 and solid solutions of Na3PO4 doped with Mg2+ and Zn2+. Na3PO4 has two solid phases: a low-temperature tetragonal phase and a high-temperature cubic phase. A factor group analysis of the two phases yielded the expected number of modes and their symmetry-based assignments. A temperature-dependent Raman spectroscopic study was then performed for the pure compound. Analysis of the doped Na3PO4 has also provided another description of the local structural distortions present in the cubic structure.