Structural phase transition in the ordered fluorides M^IIZrF₆(M^II=Co,Zn). I. Structural study

Abstract

The mixed fluorides with formula MIIZrF6 (MII=Co,Zn) exhibit an ordered ReO3-type structure: this structure is made of a three-dimensional arrangement of corner-shared octahedra with an ordered distribution of MII and ZrIV cations in the octahedra sites. These materials undergo around room temperature a first-order ferroelastic structural phase transition from a cubic phase (Fm3m) to a rhombohedral modification (R3), as established by neutron and X-ray diffraction experiments. From a structural determination of CoZrF6 at different temperatures through the phase transition, three order parameters have been measured, which enable one to describe completely the extent of lattice distortion in the rhombohedral phase: the spontaneous strain es, the coordinate of octahedra rotation around the threefold axis R and the octahedra internal deformation coordinate Q. Important thermal variations are observed for es and R, but the coordinate Q always remains very small, showing that the Co(Zr)F6 octahedra are essentially rigid bodies. X-ray diffuse scattering experiments on CoZrF6 at room temperature (cubic phase) are consistent with the existence of structural disorder due to rotations of the Co(Zr)f6 octahedra around the cubic fourfold axes.

Composition, Response to Pressure, and Negative Thermal Expansion in MIIBIVF6 (M = Ca, Mg; B = Zr, Nb)

“…At temperatures immediately above the transition, MgZrF 6 displays modest NTE (greatest magnitude at ∼175 K, α v ≈ −15 × 10 –6 K –1 ). Upon warming, the thermal expansion becomes positive, crossing through zero at close to 500 K. Diffraction data for the low-temperature phase are broadly consistent with an R 3̅ structure, as previously reported for the low-temperature form of CoZrF 6 . , However, the Rietveld fits were less than optimal, presumably due to the existence of strains in regions close to the domain boundaries below the ferroelastic transition. Upon compression at room temperature, cubic MgZrF 6 , unlike cubic CaNbF 6 , undergoes a phase transition that is associated with octahedral tilts (370 MPa) prior to undergoing a reconstructive phase transition at ∼1 GPa.…”

“…The diffraction data show evidence for phase coexistence over a narrow temperature range, suggesting that the transition is first order. The data for the low-temperature phase could be fit using a cation-ordered ReO 3 -type model with R 3̅ symmetry similar to that previously reported for CoZrF 6 , although there was clear evidence of some scattering between the newly split Bragg peaks (Figure b). Scattering of this type is quite often seen below ferroelastic phase transitions and can be attributed to strains associated with domain boundaries in the twinned low-temperature material …”

“…The literature on ReO 3 -type M II B IV F 6 indicates that mid and late 3d metals on the M site, such as Fe, Co, Ni, and Zn, destabilize the cubic structure and hence are likely to preclude strong NTE. , Phase transition temperatures have been reported to range from ∼400 K for Ni to ∼150 K for Mn in MZrF 6 , although contrary to this and other literature on CoZrF 6 , , a very recent paper on thermal expansion in MZrF 6 reports no phase transitions for the Co and Ni materials above 125 K . In this paper, we examine how the replacement of Ca 2+ and Zr 4+ by Mg 2+ and Nb 4+ affects the thermal expansion and behavior under pressure of ReO 3 -type M II B IV F 6 .…”

Mn(IV)‐Substituted Metal(II) Hexafluorido Metallates(IV): Synthesis, Crystal Structures, and Luminescence Properties

“…Despite quite similar ways of syntheses, we obtained the compounds ZnZrF 6 : Mn and ZnHfF 6 : Mn in different structure types. Unsubstituted ZnZrF 6 is known to be dimorphic, [42,46,47] so it is extremely likely that ZnHfF 6 is also dimorphic. The difference of the location of the emission maximum of ZnHfF 6 : Mn and CdHfF 6 : Mn may be due to the different structure types we obtained the compounds in or due to the differences in the chemical hardness of Zn 2+ and Cd 2+ .…”

Electron Spin Relaxation of Cu(II) Ions in ZnGeF6·6H2O Crystal with Strong Jahn-Teller Effect

“…The water and fluorine octahedra are interconnected by hydrogen bonds and there exists a disorder of GeF 6 octahedra between two positions. Ordering of the octahedra leads to the first order phase transition at about 200 K in ZnGeF 6 · 6H 2 O [9,10] and at 272 K in ZnZrF 6 [11]. Copper(II) ions introduced in the host lattice of ZnGeF 6 · 6H 2 O substitute Zn 2+ ions and produce significant distortion of the nearly perfect O h -symmetry water octahedra.…”