Structures and thermal stability of the α-LiNH₄SO₄ polytypes doped with Er³⁺ and Yb³⁺

Abstract

In order to clarify the polymorphism in the lithium sulfate family, LiRE x (NH 4 ) 1 À x SO 4 (0.5 x 4.0 mol%, nominal value; RE = Er 3+, Yb 3+and Dy 3+ ) crystals were grown from aqueous solution by slow evaporation between 298 and 313 K. The doping of the samples allowed us to obtain two polymorphic forms, and , of LiNH 4 SO 4 (LAS). By means of X-ray diffraction (XRD) in single crystals, we determined the crystal structures of two newpolytypes, which we have named 1 -and 2 -LAS. They present the same space group P2 1 /c and the following relation among their lattice parameters: a 2 = Àc 1 , b 2 = Àb 1 , c 2 = À2a 1 À c 1 . In order to evaluate the stability of the newpolytypes, we performed thermal analysis, X-ray diffraction and dielectric spectroscopy on single crystals and polycrystalline samples over the cyclic temperature range: 190 ! 575 ! 190 K. The results obtained by all the techniques used in this study demonstrate that -polytypes are stable across a wide range of temperatures and they show an irreversible phase transition to the paraelectric -phase above 500 K. In addition, a comparative study of -andpolytypes shows that both polymorphic structures have a common axis, with a possible intergrowth that facilitates their coexistence and promotes the reconstructive ! transition. This intergrowth was related to small anomalies detected between 240 and 260 K, in crystals with an -habit.

Solid-State Synthesis and Phase Transitions in the RE2(MoO4)3 Family Monitored by Thermodiffraction

“…Unlike the α-phase with a scheelite-like structure, the β-Gd 2 (MoO 4 ) 3 structure-type does not contain ...–{[LnO 8 ]­[LnO 8 ]}–{[LnO 8 ]­[LnO 8 ]}–... bands and lanthanide polyhedra form isolated dimers. To better understand the mechanisms leading to the α – β or β–α transitions, a first analysis has been performed, similar to the well-known α → β reconstructive phase transitions of LiNH 4 SO 4 crystals. , Figure shows a view containing the b (α-phase) and c (β-phase) axes along the vertical direction. We will call [A, -A, TA, -TA, A, and -A] the sequence of rows of tetrahedra of the α-phase (left) and [B, -B, TTB, B and -B] the series of rows of tetrahedra of the β-phase (right).…”

Дилатометричні дослідження кристалів LiNH4SO4 з домішкою марганцю

“…Recently, the considerable attention has been paid to the study of the physical properties of crystals belonging to the family of sulfates AB SO 4 (A, B = Li, Na, K, Rb, Cs, or NH 4 ) and possessing a number of interesting physical properties (ferroelectricity and ding on their growth conditions, they can exist in two modifications: 𝛼 and 𝛽. For pure LiNH 4 SO 4 crystals of 𝛽-modification (𝛽-LAS), their optical, dielectric, elastomechanical, and thermal properties together with the influence on them of hydrostatic or uniaxial pressure, as well as such phenomena as nuclear magnetic resonance (NMR), Brillouin scattering, and so forth, have been studied [1][2][3][4][5][6][7][8][9][10][11][12]. In particular, those crystals demonstrate a number of interesting PTs.…”