Lahcen Elammari

Crystal structures and ferroelectric properties of AIBIIPO4 (AI = monovalent cation and BII = divalent cation)

Abstract Some of the title compounds have been found to exhibit ferroelectricity at room temperature. Their structure types can be classified in three series according to the cations coordination number (CN) and size: 1.- single cation with CN 4: olivine and glaserite; 2.- two cations with CN 4: Beryllonite and stuffed tridymite; 3.- all cations with CN 4: α-LiZnPO4 is the unique case known.

A new ferroelectric phosphate family

A crystal chemical study carried out on phosphate single crystals with the formula AIBIIPO4 (AI=Na, K; BII=Ca, Zn, Sr, Cd, Ba, Pb) has permitted to follow the evolution of the crystal symmetry inside these series. Second harmonic generation (SHG) tests held on powder or small single crystals and hysteresis loops displayed on ceramic samples confirmed the existence of ferroelectricity in these material series.

Elaboration and non linear properties of ortho-phosphate solid solutions AiBII 1-xMII xPO4 (AI = monovalent cation, BII & MII = divalent cations)

Abstract X-ray crystallography has allowed to identify new solid solutions with the formulae LiPb1–xMxPO4, NaBa1–yM′yPO4 and NaPb1–zSrzPO4. When M = Sr, Ba, × varies between 0.0 and 0.4 in the vicinity of LiPbPO4 and between 0.8 and 1.0 close to LiMPO4 while for M″=Sr, 0.2 ≤y≤0.8 and M″=Pb, 0.0 ≤y≤ 1.0. The third system corresponds to a continuous solid solution between z=0 and z=0.8. Phase transitions were determined along the solid solutions investigated. Second harmonic generation tests has shown that the highest yield frequency doubling was recorded for compounds of the systems LiPbPO4-LiMPO4 (M=Sr, Ba).

Crystal structure of LiPbPO4 with “Li4O13” cluster

Abstract The structure of LiPbPO4 was solved at 173 K in the orthorhombic space group Pna21 and unit cell parameters a = 7.969(3), b = 18.591(5), and c = 4.926(1) A, V = 730(1)A3, F(000) = 1056, μ = 469 cm−1, Dm = 5.55 g · cm−3, Dc = 5.63 g · cm−3, and Z = 8. The structure was refined to a final R = 0.0295 for 1872 independent reflections. It consist of PO4 and LiO4 distorted tetrahedra which are linked together in such a way that wide tunnels are formed where the lead cations are located.

Growth of CaKPO4 Crystals from a K2MoO4 Flux.

Colorless and transparent crystals of CaKPO4 were grown for the first time from a K2MoO4 flux by a slow cooling method. Flux growth of CaKPO4 crystals was conducted by heating mixtures containing 2to 20 mol% solute at 1100°C for 10 h, followed by cooling to 500°C at a rate of 5°C h-1. CaKPO4 crystals of sizes up to 0.5 mm x 0.5 mm×0.1 mm were grown. The crystal sizes were dependent on the solute content. The most suitable content was 4 mol%. The shape of the crystals was a hexagonal thin plate. On the basis of X-ray diffraction and differential thermal analysis studies, it was shown that the grown crystals could exist in two polymorphic forms. The stable form at room temperature transformed into high-temperature form (hexagonal) at about 660°C. The most developed crystal faces of the hightemperature form had an index of [0001].