The crystal structures of pressure-induced LiSrAlF₆-II and LiCaAlF₆-II

Abstract

The crystal structures of LiCaAlF 6 -II and LiSrAlF 6 -II (both P2 1 /c, Z = 4) occurring at high pressures and room temperature were studied with synchrotron angle-dispersive x-ray powder diffraction in diamond anvil cells. The structure of LiSrAlF 6 -II stable between 1.6 and 3.0 GPa was solved with a global optimisation algorithm and group theory considerations, and refined with the Rietveld method in the rigid-body approximation. It is a distorted variant of the ambient pressure polymorph (LiSrAlF 6 -I, P 31c, Z = 2), in which each cation occupies a deformed octahedral site. LiCaAlF 6 transforms to this monoclinic polymorph II above about 7 GPa. The differences in the highpressure behaviours of LiCaAlF 6 and LiSrAlF 6 are discussed by considering the ionic radii.

Time-resolved spectroscopy in LiCaAlF6 doped with Cr3+ as a function of pressure and temperature

“…A trigonal to monoclinic PT takes place in LiCaAlF 6 upon pressure around 7 GPa [8]. The pressure-induced PT is clearly evidenced by PL using Cr 3+ as spectroscopic probe.…”

“…The PT has important consequences in the PL properties. Firstly, the PL band experiences an abrupt redshift upon pressure from phase I to phase II causing an increase of the critical pressure required to attain ESCO [8]. Secondly, the narrow feature associated with 2 E-4 A 2 emission which is observed at 6 GPa disappears above the PT at 7 GPa.…”

“…In fact LiCaAlF 6 : Cr 3+ provides the highest CF at the Al site for investigating ESCO phenomena [7]. In addition, the host material exhibits a first-order structural phase transition (PT) from trigonal (P-31c: phase I) to monoclinic (P2 1 /c: phase II) between 7 and 9 GPa [8], which reduces the Cr 3+ site symmetry yielding important PL changes. The effects of temperature and pressure, particularly the pressure-induced PT and ESCO phenomena, on the PL properties of LiCaAlF 6 : Cr 3+ are investigated.…”

Dilithium zirconium hexafluoride Li2ZrF6 at high pressures: A new monoclinic phase

“…In the context of the P-T phase diagram for the A 2 BX 6 compounds, the behaviour of ordered derivatives of the Li 2 ZrF 6 type (P 31m, ZZ1) [7,8] provides additional information. The structure of LiSrAlF 6 -II (P2 1 /c, ZZ4), stable between 1.6 and 3.0 GPa, is a distorted derivative of the ambient pressure polymorph (LiSrAlF 6 -I, P 31c, ZZ2) with the cations exclusively in deformed octahedral coordinations [13]. LiCaAlF 6 transforms to this monoclinic polymorph II above about 7 GPa.…”

“…Our further interest in the high-pressure behaviour of Li 2 ZrF 6 arises from the fact that it is a disordered archetype of the colquiriite LiM 0 M 00 F 6 compounds (P 31c, ZZ2; M 0 ZCa or Sr; M 00 ZAl, Ga, or Cr) considered for optical applications [7,13,14]. The understanding of pressure-induced phase transitions and structures of the A 2 BX 6 materials could be improved by in situ investigations of the transformations, involving the changes in coordination spheres of the cations and packing of the anions.…”