In situhigh-pressure synchrotron x-ray diffraction study ofSc2W3O12at up to 10 GPa

Abstract

Sc 2 W 3 O 12 is known to display both negative thermal expansion and significant Sc 3+ conductivity. Monochromatic synchrotron x-ray powder diffraction was used to study Sc 2 ͑WO 4 ͒ 3 at pressures up to 10 GPa in diamond anvil cells. Nitrogen and isopropanol were both used as pressure transmitting media. A phase transition from orthorhombic ͑Pnca͒ to monoclinic ͑P2 1 / a͒ symmetry was observed at ϳ0.3 GPa with a volume reduction of ϳ1.7%. A second crystalline to crystalline phase transition was seen at ϳ2.8 GPa. The latter was not reversible on decompression. The orthorhombic phase displays highly anisotropic compressibility, but that of the monoclinic phase is almost isotropic ͓orthorhombic in isopropanol ␤ a = 6.32͑16͒ ϫ 10 −3 GPa −1 , ␤ b = 1.76͑21͒ ϫ 10 −2 GPa −1 , ␤ c = 6.84͑32͒ ϫ 10 −3 GPa −1 ; monoclinic in isopropanol ␤ a = 1.85͑3͒ ϫ 10 −2 GPa −1 , ␤ b = 1.78͑3͒ ϫ 10 −2 GPa −1 , ␤ c = 1.69͑9͒ ϫ 10 −2 GPa −1 ͔. Sc 2 ͑WO 4 ͒ 3 is very soft even when compared to most other NTE phases. Fitting a Birch-Murnaghan equation of state to data from the experiment in isopropanol gave a bulk modulus ͑K 0 ͒ of 31͑3͒ for the orthorhombic and 14͑1͒ GPa for the monoclinic phase. This reduction in bulk modulus at the phase transition and the reduction in volume per formula unit is almost entirely associated with changes in the orthorhombic a and c directions. The variation of b ortho and its monoclinic equivalent lattice constant is essentially continuous through the transition. The pressure of the orthorhombic to monoclinic phase transition in A 2 M 3 O 12 compounds does not have a strong dependence on the identity of the A 3+ cation. Amorphization did not occur in the pressure range studied.

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

“…The low densities and highly flexible frameworks of many NTE materials, combined with the presence of lattice modes that soften on volume reduction [10][11][12][13][14][15][16] predispose them to interesting highpressure behavior. Crystalline to crystalline phase transitions have been observed at high pressure in cubic ZrW 2 O 8 [11,12,[17][18][19][20], cubic HfW 2 O 8 [13,21], cubic ZrMo 2 O 8 [22,23], cubic HfMo 2 O 8 [22], ZrV 2 O 7 [24], Sc 2 W 3 O 12 [25], Sc 2 Mo 3 O 12 [26,27] and Al 2 W 3 O 12 [27][28][29]. Additionally, pressure induced amorphization (PIA) has been seen for ZrMo 2 O 8 [22,23], ZrW 2 O 8 [30], Sc 2 W 3 O 12 [31], Sc 2 Mo 3 O 12 [32] and Lu 2 W 3 O 12 [33] at low pressures.…”

Structural changes accompanying negative thermal expansion in Zr2(MoO4)(PO4)2

“…Many compositions are known to form with structures related to that of orthorhombic Sc 2 W 3 O 12 [1][2][3][4]. They typically display anisotropic volume NTE while orthorhombic, but are susceptible to symmetry lowering phase transitions [5], with a concomitant loss of NTE, on cooling or modest compression [6][7][8][9]. The Sc 2 W 3 O 12 framework is amenable to a wide range of substitutions beyond the simple replacement of Sc 3+ with other trivalent ions; for example, materials such as A 2 (MO 4 )(PO 4 ) 2 (A-Zr or Hf; M-W or Mo) [10][11][12][13], (HfMg)(WO 4 ) 3 [14][15][16][17], and Zr 2 (SO 4 )(PO 4 ) 2 [18] have been reported to have related structures.…”

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

“…This agrees with a previous Raman study by Torres Dias et al (2013), which suggested that Pbcn-Y 2 Mo 3 O 12 underwent a transition to a lower symmetry phase below 0.3 GPa. It is not surprising that this transition would occur at such a low pressure, as previous studies have shown that Sc 2 W 3 O 12 (Garg et al, 2005a; Varga et al, 2005b, 2006; Cetinkol et al, 2008), Sc 2 Mo 3 O 12 (Varga et al, 2005a) and Al 2 W 3 O 12 (Varga et al, 2005a) behave similarly and undergo this transition at 0.3 GPa, 0.25 GPa and 0.1 GPa, respectively. Initial unit cell parameters for the corresponding monoclinic unit cell were estimated based on the known transformation matrix (Evans and Mary, 2000), and combined with the atomic coordinates of α−Fe 2 Mo 3 O 12 as a starting model for a Rietveld refinement.…”

“…In contrast, the Cr 2 Mo 3 O 12 peaks start to become progressively diffuse above 5.5 GPa (scan 54), indicating the onset of disorder or amorphization. Many NTE materials have been reported to amorphize under pressure (Huang, 1998; Perottoni and da Jornada, 1998; Garg et al, 2001, 2005b; Liu et al, 2001, 2002; Secco et al, 2001, 2002a,b; Arora et al, 2004, 2005; Karmakar et al, 2004; Mukherjee et al, 2004; Paraguassu et al, 2004; Varga et al, 2005b; Gates et al, 2006; Keen et al, 2007; Catafesta et al, 2008; Cetinkol et al, 2009; Baiz et al, 2012; Torres Dias et al, 2013; Salke et al, 2018), especially under non-hydrostatic conditions, as their open frameworks allow for volume-reducing rotations of the constituent polyhedra. Under non-hydrostatic pressure, such random reorientations can become “frozen in” at relatively low pressures, resulting in progressive loss of long range order.…”

“…As such, the high pressure behavior of A 2 M 3 O 12 materials remains unpredictable. One exception to this has been the observation that compositions adopting the orthorhombic structure at room temperature undergo a phase transition to the slightly denser P2 1 /a-structure below 0.5 GPa (Paraguassu et al, 2004; Garg et al, 2005a; Varga et al, 2005a,b, 2006; Cetinkol et al, 2008; Varga, 2011; Lind, 2012; Maczka et al, 2012). Generally, at least one additional phase transition occurs at higher pressures between 1 and 4 GPa.…”