pH Controlled Pathway and Systematic Hydrothermal Phase Diagram for Elaboration of Synthetic Lead Nickel Selenites

Abstract

The PbO−NiO−SeO 2 ternary system was fully studied using constant hydrothermal conditions at 473 K. It yields the establishment of the corresponding phase diagram using a systematic assignment of reaction products by both powder and single-crystal X-ray diffraction. It leads to the preparation of three novel lead nickel selenites, α-PbNi(SeO 3 ) 2 (I), β-PbNi(SeO 3 ) 2 (II), and PbNi 2 (SeO 2 OH) 2 (SeO 3 ) 2 (III), and one novel lead cobalt selenite, α-PbCo(SeO 3 ) 2 (IV), which have been structurally characterized. The crystal structures of the α-forms I, IV, and III are based on a 3D complex nickel selenite frameworks, whereas the β-PbNi(SeO 3 ) 2 modification (II) consists of nickel selenite sheets stacked in a noncentrosymmetric structure, second-harmonic generation active. The pH value of the starting solution was shown to play an essential role in the reactive processes. Magnetic measurements of I, III, and IV are discussed. ■ INTRODUCTIONThe demand for specific noncentrosymmetric (NCS) crystal structures is very strong because interesting physical properties, such as second-harmonic generation (SHG), 1 piezoelectricity, 2 ferroelectricity, 3 and pyroelectricity, 4 can be expected. In particular, nonlinear optic materials have become tremendously important and are drawing more and more attention owing to their promising applications in laser science and technology. The NCS compounds are commonly observed when dealing with (Ti 4+ , V 5+ , Mo 6+ ) octahedrally coordinated d 0 transition metals susceptible to second-order Jahn−Teller (SOJT) effects. Typical SOJT asymmetric coordination environments are also favored for cations with a stereoactive lone pair of electrons (As 3+ , Se 4+ , Sb 3+ , Pb 2+ , and Bi 3+ , etc.) in more drastic manner leading to a diversity of unusual polyhedral coordination. The crystal chemistry of metal selenites is very rich as first reviewed by Verma. 5 The incorporation of heavy lone pair cations such as Pb 2+ /Bi 3+ with selenite groups increases the chance to achieve asymmetric building units or NCS crystal structures as demonstrated in several earlier works dedicated to the exploration of PbO/Bi 2 O 3 −MO x −SeO 2 (M = Cu 2+ , V 3+ , Ge 4+ , Nb 5+ , V 5+ , Mo 6+ , W 6+ ; x = 1, 2; y = 1, 2, 3, 5) ternary systems. 6−11 From the experimental viewpoint, due to the low and close melting and sublimation temperatures (340 and 350°C, respectively), the crystal growth of selenites is generally achieved using either chemical transport routes, 12,13 recently leading to original bismuth selenites, including giant cell compounds, 14 or hydrothermal methods due to the good solubility and reactivity of (SeO 3 ) 2− anions. However, as recently demonstrated by the evidence of a number of phases in competition in the CoSeO 3 − SeO 2 −H 2 O phase diagram 15 and other hybrid inorganic− organic systems, 16,17 a systematic approach may be preferred for exhaustive search of novel crystal structures. Thus, research was not yet carried out on ternary systems with lead and nickel/ cobalt as transition...

Chemically Induced Polytypic Phase Transitions in the Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) System

“…The structural complexity parameters, which are estimated as a Shannon information content per atom ( I G ) and per unit cell ( I G,total ), were calculated to analyze structural variations occurring due to the Se-for-S substitution. According to this approach recently reported in refs − and applied in refs , , , and − , complexity of the crystal structure can be determined by the amount of Shannon information it contains and measured in bits per atom and per unit cell. The amount of Shannon information reflects multiplicity and the number of different crystallographic sites in a reduced unit cell of a structure.…”

Crystal Chemistry and Structural Complexity of Natural and Synthetic Uranyl Selenites

“…The other two organically templated uranyl selenites (33 [48] and 36 [50]) were obtained during evaporation at room temperature from the aqueous solution of UO 2 (NO 3 ) 2 ·6H 2 O, SeO 2 , and respective amine. Since SeO 2 transforms to weak selenous acid in water, the low dissociation ability of the latter [62] and the presence of [NO 3 ] − groups in the structures of both compounds explains the long crystallization process of 1 to 2 months. It is likely that dissociation of uranyl nitrate and the presence of amine finally helped to create an environment sufficient for the selenous acid dissociation, and thus to start the crystallization of uranyl selenites.…”

Embreyite: structure determination, chemical formula and comparative crystal chemistry

“…Apical vertices of triangular selenite Se 4+ O 3 pyramids are oriented into the centre of the layers identically to (Cr,P)O 4 groups in embreyite (Fig. 6 e ) in the crystal structures of K 2 M (SeO 3 ) 2 compounds [ M 2+ = Mg (Hesse and Simons, 1982), Co (Wildner, 1992 a ) and Mn (Wildner, 1992 b )], whereas the apices of the pyramids are aligned one by one in the same direction emphasizing a non-centrosymmetric character in β-PbNi(SeO 3 ) 2 (Kovrugin et al ., 2015) and RbSc(SeO 3 ) 2 (Song and Ok, 2015) (Fig. 6 f ).…”