Skip to Main Content
Skip Nav Destination
GEOREF RECORD

Temperature dependence of crystal structure of CaGeO (sub 3) high-pressure perovskite phase and experimental determination of its Debye temperatures studied by low- and high-temperature single-crystal X-ray diffraction

Akihiko Nakatsuka, Shoichi Kuribayashi, Noriaki Nakayama, Hiroshi Fukui, Hiroshi Arima, Akira Yoneda and Akira Yoshiasa
Temperature dependence of crystal structure of CaGeO (sub 3) high-pressure perovskite phase and experimental determination of its Debye temperatures studied by low- and high-temperature single-crystal X-ray diffraction
American Mineralogist (June 2015) 100 (5-6): 1190-1202

Abstract

Single-crystal X-ray diffraction study of CaGeO (sub 3) perovskite has been conducted over the temperature range of 98 to 1048 K. The crystal begins to deteriorate at a temperature above about 900 K and completely amorphizes by 980 K. The diffraction-intensity distribution and the structure refinements indicate that the Pbnm structure is kept until the occurrence of amorphization. The obtained unit-cell parameters, unit-cell volumes, bond lengths, and displacement parameters increase monotonously with increasing temperature. Thus, no evidence for the existence of the Cmcm high-temperature phase, previously suggested above 520 K, is observed. The Ge-O bond lengths show much smaller thermal expansions than the Ca-O bond lengths; the former ranges between 0.42(2) X 10 (super -5) K (super -1) and 0.57(2) X 10 (super -5) K (super -1) , and the latter between 1.58(4) X 10 (super -5) K (super -1) and 3.96(6) X 10 (super -5) K (super -1) . The Debye temperatures and static disorder components for each constituent atom were determined by applying the Debye model to the temperature dependence of mean square displacements (MSDs) of the atoms. Consequently, no significant static disorder components can be detected in each atom. The Debye temperatures averaged over all directions, obtained from the Debye model fitting to U (sub eq) , yield the harmonic one particle potential coefficients of 4.76(2) eVAa (super -2) for Ca, 11.0(1) eVAa (super -2) for Ge, 5.02(2) eVAa (super -2) for O1, and 5.33(5) eVAa (super -2) for O2. These values become larger in order of Ca < O1 < O2 << Ge, which shows that the one particle potential of Ge is much narrower than that of Ca. This relationship between Ca and Ge is consistent reasonably with bonding stiffness expected from the thermal expansion coefficients of the bond lengths. The anisotropies of MSDs are remarkable in O1 and O2 atoms as a consequence of the strong interaction with adjacent Ge atoms, forming the rigid bonds with these O atoms.In comparison of the three Pbnm orthorhombic perovskites of CaGeO (sub 3) , CaTiO (sub 3) , and MgSiO (sub 3) , all of these have the BO (sub 6) octahedra more rigid than the AO (sub 12) polyhedra (A = Ca or Mg; B = Ge, Ti, or Si) and the tilt angles of BO (sub 6) octahedra are the largest in MgSiO (sub 3) perovskite. These observations indicate that if MgSiO (sub 3) perovskite under high pressures undergoes the same sequence of the high-temperature phase transitions as CaTiO (sub 3) perovskite, the phase boundaries have positive Clapeyron slopes and the phase transition temperatures should become further much higher than those (1512 K for the Pbnm to I4/mcm transition and 1635 K for the I4/mcm to Pm3m transition) observed in CaTiO (sub 3) perovskite at ambient pressure. This leads to the conclusion that the high-temperature phase transition to a perovskite phase with different symmetry under high pressures previously suggested in MgSiO (sub 3) perovskite is unlikely.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 5-6
Title: Temperature dependence of crystal structure of CaGeO (sub 3) high-pressure perovskite phase and experimental determination of its Debye temperatures studied by low- and high-temperature single-crystal X-ray diffraction
Affiliation: Yamaguchi University, Graduate School of Science and Engineering, Ube, Japan
Pages: 1190-1202
Published: 201506
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 44
Accession Number: 2015-065522
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
Secondary Affiliation: University of Hyogo, JPN, JapanTohoku University, JPN, JapanOkayama University, JPN, JapanKumamoto University, JPN, Japan
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201528
Close Modal

or Create an Account

Close Modal
Close Modal