High Pressure Behavior of Chromium and Yttrium Molybdate (Cr2Mo3O12, Y2Mo3O12)

Abstract

The high pressure behavior of negative thermal expansion materials continues to be of interest, as their potential use in controlled thermal expansion composites can be affected by irreversible pressure-induced phase transitions. To date, it is not possible to predict the high pressure behavior of these compounds, necessitating measurements on each composition. In this work, high pressure synchrotron powder X-ray diffraction studies of Cr2Mo3O12 and Y2Mo3O12 were conducted in a diamond anvil cell. Chromium molybdate, which adopts the monoclinic P21/a structure under ambient conditions, was found to not undergo any crystalline-crystalline transitions up to 8.9 GPa. The orthorhombic ambient pressure polymorph of yttrium molybdate was found to undergo a phase transition to the monoclinic P21/a scandium tungstate structure below 0.13 GPa. This structure is frequently observed for related materials at low temperatures, but has never been reported for Y2Mo3O12. No additional changes in this material were observed up to 4.9 GPa. The fact that the monoclinic polymorphs of these materials do not undergo phase transitions within the studied pressure range makes them unique among A2M3O12 materials, as most isostructural compositions undergo at least one phase transition to crystalline high pressure phases.

Negative and Near-Zero Thermal Expansion in A2M3O12 and Related Ceramic Families: A Review

“…A direct reduction of the bulk modulus by entropic elasticity (i.e., an increase in entropy upon compressive strain) has been found in floppy networks, including ScF 3 (Tkachenko and Zaliznyak, 2021), indicating an underlying mechanism that reduces the bulk modulus in flexible frameworks. This relationship is not absolute (e.g., α-ZrW 2 O 8 is stiffer than the thermomiotic A 2 M 3 O 12 oxides and has a more negative CTE (Drymiotis et al, 2004;Varga et al, 2006;Young et al, 2018)), because the Grüneisen parameters of thermomiotic materials vary considerably, and because of the effects of anisotropy, which require further discussion.…”

“…Note that A 2 M 3 O 12 and related filler materials are frequently prone to pressure-induced transformation from the desired orthorhombic phase to the monoclinic phase at pressures <250 MPa (Varga et al, 2005), with some exceptions such as Zr 2 WP 2 O 12 which is still orthorhombic at pressures as high as 300 MPa . Such transformation must be considered in selecting the compaction pressure value for the green CMCs prior sintering (Young et al, 2018). The compaction pressures reported for these CMCs (see Table 1), ranged between 50 and 245 MPa, and did not induce the unwanted phase transition.…”

Structures and Photophysical Characterization of the Samarium-doped Y2Mo3O12 Ceramics

“…In order to understand the NTE mechanisms in Y 2 Mo 3 O 12 , several studies have been performed: the relation between AO 6 polyhedral distortion and NTE in orthorhombic Y 2 Mo 3 O 12 [16], and the effect of water species on the phonon modes [11,17]. There are many studies to improve the NTE properties in Y 2 Mo 3 O 12 [18,19], and to characterize various properties of Y 2 Mo 3 O 12 [20][21][22]. These studies have shown that the hygroscopicity is related to the sizes of cations A, and the reduction of hygroscopicity is achieved by substituting trivalent cations to Y 3+ sites [10,23].…”