Composition, Response to Pressure, and Negative Thermal Expansion in M^IIB^IVF₆ (M = Ca, Mg; B = Zr, Nb)

Abstract

CaZrF6 has recently been shown to combine strong negative thermal expansion (NTE) over a very wide temperature range (at least 10–1000 K) with optical transparency from mid-IR into the UV range. Variable-temperature and high-pressure diffraction has been used to determine how the replacement of calcium by magnesium and zirconium by niobium­(IV) modifies the phase behavior and physical properties of the compound. Similar to CaZrF6, CaNbF6 retains a cubic ReO3-type structure down to 10 K and displays NTE up until at least 900 K. It undergoes a reconstructive phase transition upon compression to ∼400 MPa at room temperature and pressure-induced amorphization above ∼4 GPa. Prior to the first transition, it displays very strong pressure-induced softening. MgZrF6 adopts a cubic (Fm3̅m) structure at 300 K and undergoes a symmetry-lowering phase transition involving octahedral tilts at ∼100 K. Immediately above this transition, it shows modest NTE. Its’ thermal expansion increases upon heating, crossing through zero at ∼500 K. Unlike CaZrF6 and CaNbF6, it undergoes an octahedral tilting transition upon compression (∼370 MPa) prior to a reconstructive transition at ∼1 GPa. Cubic MgZrF6 displays both pressure-induced softening and stiffening upon heating. MgNbF6 is cubic (Fm3̅m) at room temperature, but it undergoes a symmetry-lowering octahedral tilting transition at ∼280 K. It does not display NTE within the investigated temperature range (100–950 K). Although the replacement of Zr­(IV) by Nb­(IV) leads to minor changes in phase behavior and properties, the replacement of the calcium by the smaller and more polarizing magnesium leads to large changes in both phase behavior and thermal expansion.

Negative Thermal Expansion Design Strategies in a Diverse Series of Metal–Organic Frameworks

“…Selected metal fluoride values were taken from ref. [45] . UiO-66(Zr) and UiO-67(Zr) values from this work are for the frameworks having undergone a 95 C heat pre-treatment.…”

Negative X-ray expansion in cadmium cyanide

“…It now joins the small group of materials, including CaNbF 6 and the canonical ZrW 2 O 8 with capacities greater than 2%. [49][50][51][52]…”

Zero thermal expansion in cubic MgZrF6

“…It is well known that most materials expand on heating due to the anharmonicity of atomic potentials. Interestingly, an increasing quantity of negative thermal expansion (NTE) materials, which contract with increasing temperature, have been reported in diverse solids, like ZrW 2 O 8 , 1 ScF 3 , 2 M II B IV F 6 , [3][4][5] and MOF. 6 The nature of NTE phenomenon has been revealed to be associated with various factors, like magnetic transition in Invar Fe-Ni alloys, 7 spontaneous volume ferroelectrostriction (SVFS) in PbTiO 3 -based ferroelectrics, 8 intermetallic charge transfer in BiNiO 3 , 9 phonon-related transverse cooperative vibration in ReO 3type fluoride, 10 geometrically flexible diatomic linkages in Prussian blue families, 11 and magnetovolume effect (MVE) in Mn 3 AX antiperovskite.…”