Structures and phase transitions of trigonal ZrMo₂O₈ and HfMo₂O₈

Abstract

This paper describes the structures, thermal-expansion properties and phase transitions of the trigonal forms of ZrMo(2)O(8) and HfMo(2)O(8). Both phases adopt a P(-)3m structure at room temperature and show positive thermal expansion. Both phases also undergo a displacive phase transition at high temperature (ZrMo(2)O(8) at 487 K and HfMo(2)O(8) at 463 K) to a higher-symmetry structure that has lower thermal expansion. The structure of the high-temperature alpha'-AMo(2)O(8) form (A = Zr and Hf) has been refined from powder diffraction data in space group P(-)3m1.

Pressure-induced amorphization of cubic ZrMo2O8 studied in situ by X-ray absorption spectroscopy and diffraction

“…Pressure induced transformations in NTE materials are not purely of academic interest, the changes in expansion characteristics that accompany them can be detrimental to the material's applications as the processing and use of composites can involve high pressures [5][6][7]. ZrMo 2 O 8 is polymorphic with trigonal [42], monoclinic [43], orthorhombic [44] and cubic forms [45,46] accessible at ambient temperature using ambient pressure syntheses. There are also a variety of high-pressure ZrMo 2 O 8 phases known [24,47,48].…”

Phase Behaviors of the ZrMo2–xWxO8(x= 0.2–2.0) System and the Preparation of an Mo‐Rich Cubic Phase

“…The pure trigonal ZrMo 2-x W x O 8 (x = 0.2-0.7) was prepared at 782°C and was indexed by using the αЈ-ZrMo 2 O 8 model of space group P3m1 [13] rather than α-ZrMo 2 O 8 of space group P31c, [14] because of the absence of 201, 103, and 211 diffractions in the XRD pattern of trigonal Table 2. The stability of cubic ZrMo 2-x W x O 8 solid solutions (x = 0.2-0.8) increases rapidly with increasing W-fraction (x) from 0.2 to 0.8.…”

Ligand Association/Dissociation Paths and Ill‐Defined Coordination Numbers

“…One of the A atoms in compounds with the A 2 M 3 O 8 nolandite structure is also along the pathway that goes from the tetrahedron to the capped octahedron. In ZrA C H T U N G T R E N N U N G (MoO 4 ) 2 , [64] with the same structural type as MgA C H T U N G T R E N N U N G (AlH 4 ) 2 , the Mo atom is practically unperturbed from the tetrahedral geometry, even if there are also three Mo···O contacts through face centers (3.48 ).…”