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...