Yukio SATO, Yasuhisa YAMAMURA, Kazuya SAITO and Yuichi IKUHARA
Solids usually expand when they are heated. This is quite common behavior of solids; however, there are some exceptions. Zirconium tungstate (ZrW2O8) is a prototype material among them, because it has the highest degree of negative thermal expansion (NTE) over broad temperature range. Intensive investigation of NTE mechanisms has suggested the importance of metal−oxygen polyhedra. However, most of the studies have been done with volume-averaged techniques, and microscopic information has been lacking. Here, our electron microscopy observations have unraveled the real-space distribution of local WO4 tetrahedra ordering for the first time. We have found that (i) the WO4 ordering is partly inverted; (ii) WO4 is disordered on the nanoscale; and (iii) doping with scandium enhances the WO4 disordering. These findings led to construction of a microstructure model for ZrW2O8, providing a new structural perspective for better understanding of local structure and its role in phase transitions.
(J. Am. Chem. Soc., 134, 13942 (2012))
Yasuhisa YAMAMURA and Kazuya SAITO
Thermodynamic approach toward understanding Negative Thermal Expansion (NTE) mechanism is described. Three characteristics in lattice vibrations of NTE ceramics, arising from framework structure, were identified: low-energy mode, high-energy mode, and wide gap in between. On this basis, a concept "potential NTE material" was proposed for developing materials with NTE property in a wide temperature range.
(Bull. Ceram. Soc. Jpn., 46, 922 (2011), review in Japanese)
Yasuhisa YAMAMURA, Aruto HORIKOSHI, Syuma YASUZUKA, Hideki SAITOH and Kazuya SAITO
ZrV2O7 and HfV2O7, which show the negative thermal expansion (NTE) in the high-temperature phase, were investigated using x-ray diffraction and heat capacity calorimetry. Two sharp anomalies due to successive phase transitions were observed in temperature dependence of heat capacity at 345.5 K and 373.4 K for ZrV2O7 and 341.8 K and 370.3 K for HfV2O7. The smallness of their combined entropies of transition suggested that the phase transitions are of displacive type. Effective phonon densities of states (DOS) described by a simple model, and mode-Grueneisen parameters of the low-temperature phase were obtained through the spectrum analyses of heat capacities of ZrV2O7 and HfV2O7. Their effective phonon DOSfs shows the three features common to the NTE compounds; low-energy phonon mode, high-energy phonon mode, and wide phonon gap in between. The mode-Grueneisen parameter of low-energy modes corresponding to translational and librational vibrations of the constituent polyhedra is negative but with a small absolute value due to the distortion of V2O7 group in the low-temperature phase, resulting in positive thermal expansion. It is revealed that the release of the structural distortion upon the successive phase transitions with large volume increase leads to the NTE of ZrV2O7 and HfV2O7 in the high-temperature phase.
(Dalton Trans., 40, 2422 (2011))
Yasuhisa YAMAMURA, Satoaki IKEUCHI and Kazuya SAITO
To study the relation between the structural characteristics and the phonon property in the negative thermal expansion (NTE) compounds, the heat capacities of Sc2W3O12 and Sc2Mo3O12 were measured. Spectrum analysis of heat capacity provided their effective phonon densities of states (DOS). The DOS of Sc2W3O12 shows three features; low-energy phonon modes with negative mode-Gruneisen parameter (gammai) around 5 meV, high-energy phononmodes, and separation of phononDOS into two regions with a wide gap. The relative contribution of gammai Ci, where Ci is heat capacity of each vibrational mode i, reveals that the low-energy phonon modes with negative gammai cause the NTE and that the latter two features are necessary to maintain the NTE in a wide temperature range. Sc2Mo3O12 has the low-energy mode with the negative gammai. This fact indicates that Sc2Mo3O12 potentially has the NTE property even in its low-temperature phase showing positive thermal expansion. A comparison of the phonon DOS with other oxides shows that the phonon features are common in the NTE oxides and related to their common chemical and structural characteristics, "strong bond" and "framework structure". This finding gives us an important guide to search for new actual and/or potential NTE compounds.
(Chem. Mater., 21, 3008 (2009))
Yasuhisa YAMAMURA and Kazuya SAITO
To study the phonon properties of beta-ZrW2O8 showing the negative thermal expansion (NTE), the heat capacities of Zr1-xMxW2O8-y (M = Sc, Lu; x = 0:02, 0.04) at low temperatures were measured and analyzed. The analysis presents the effective phonon density of states (DOS) of beta-ZrW2O8, showing a rounded form around 5 meV. The rounded phonon DOS of ZrW2O8 is in marked contrast to that of the low-temperature phase of ZrW2O8, and their distinction is consistent with the difference in NTE nature between two structures.
(J. Phys. Soc. Jpn., 76, 123603 (2007))
Yasuhisa YAMAMURA, Toshihide TSUJI, Kazuya SAITO and Michio SORAI
Heat capacity of ZrW2O8 was measured in the (1.8 to 483) K temperature range by using two adiabatic and a commercial relaxation calorimeters. Standard thermodynamic functions of ZrW2O8 were estimated from the measured molar heat capacity. A large lambda-type anomaly of heat capacity due to an order-disorder phase transition was observed at T = (440 +- 0.5) K. The enthalpy and entropy of phase transition were determined to be (1.56 +- 0.01) kJ mol-1 and (4.09 +- 0.03) J K-1 mol-1, respectively. The magnitude of the entropy of transition supports the order-disorder mechanism previously suggested by the present authors.
(J. Chem. Thermodyn., 36, 525 (2004))
Heat capacities of Zr1-xHfxW2O8 (x = 0.25, 0.5 and 0.75) were measured from 1.8 to 70 K. The heat capacity of Zr increased with increasing Hf content due to atomic mass effect. Frequency distributions of lattice vibrations were estimated through an analysis of the heat capacities for Zr1-xHfxW2O8 and indicated the presence of two Einstein modes in low-energy region. The two characteristic Einstein temperatures lineraly depended on Hf content. Raman spectra of Zr1-xHfxW2O8 at room temperature showed two characteristic optical phonon modes corresponding to the two Einstein modes obtained from heat capacity analyses. Mode assignment is made of these characteristic optical phonon modes involved in the negative thermal expansion.
(J. Ceram. Soc. Jpn., Suppl. 12-1, S291 (2004))
Heat capacities of ZrW2O8 and HfW2O
8 were measured between 1.8 and 330 K. Although the two heat capacity curves are very similar, they cross around 220 K. At low temperature, the heat capacity of HfW2O8 is larger than that of ZrW
2O8. Raman spectra of the compounds were recorded at
room temperature. Frequencies of some optical phonon modes with the
large negative mode Gruneisen parameter are different between ZrW 2
O8 and HfW2O8. Frequency distribution
of lattice vibrations were obtained through an analysis of the heat capacities
of ZrW2O8 and HfW2O8. It
is found that the changes in the frequency distribution arise from the difference
in the mass and ionic radius and cause the different temperature dependence
of heat capacities. The origin of the optical phonon modes with the
large negative mode Gruneisen parameter are discussed.
(Phys. Rev. B, 66, 014301 (2002))
Heat Capacity of HfW2O8 was measured from 1.8 to
330 K, for the first time. Gruneisen function of HfW2O8
was obtained between 80 and 330 K from the heat capacity and other thermal
data. Effective phonon density of states was determined through an analysis
of the heat capacity of HfW2O8 assuming one Debye,
two Einstein and two box-shaped type functions. The mode Gruneisen parameters
were estimated. It is found that two low-energy Einstein modes at 3-6 meV
have large negative mode Gruneisen parameters, resulting in the negative
(Solid State Commun., 121, 213 (2002))
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