Synthesis, Characterization And Thermochemistry Of Hydrated Lead Borate

There are many kinds of borates with complex structures, which provide rich resources for searching useful borates. Lead borates are of special interest in the search for materials, since some of these compounds have pronounced nonlinear-optical properties,such as PbB4O7, Pb2[B5O9](OH)·H2O, Pb5B3O8(OH)3·H2O, Pb3B9O16(OH)·B(OH)3 and Pb6B11O18(OH)9.So the studies of lead borates have attracted much attention in recent years.Thermodynamic properties play very important roles in scientific research and industrial applications.As for the thermochemistry of main group borates,the standard molar enthalpies of formation of many alkaline and alkaline-earth metal borates have been measured. However,studies of the thermochemistry of the lead borates are not reported in the literature. As part of the continuing study of metal borates,the synthesis and thermochemistry of lead borates have been investigated.Four lead borates, Pb(BO2)2·H2O, PbB4O7·4H2O, Pb4B10O19·2.5H20 and Pb6B11O18(OH)9,have been synthesized by using solution method at different temperatures and hydrothermal reaction.All of the products have been identified and characterized by XRD,FT-IR,simultaneous DTA-TG and chemical analysis,which have been proved to be the pure samples and suitable for the calorimetric experiments.The enthalpies of solution of these four synthesized pure lead borates in 1 mol·L-HN03(aq), the enthalpy of solution of H3BO3(s) in 1 mol·L-1HNO3(aq) and the molar enthalpy of solution of PbO(s) in (HNO3+H3BO3)(aq) were determined by heat conduction microcalorimeter, respectively. From these data and with incorporation of the enthalpies of formation of PbO(s), H3BO3(s) and H2O(l),the standard molar enthalpies of formation of -(1820.5±1.8) kJ·mol-1 for Pb(BO2)2·H2O,-(4038.1±3.4) kJ·mol-1 for PbB4O7·4H2O,-(8231.4±8.6) kJ·mol-1 for Pb4B10O19·2.5H2O and-(9967.5±9.8) kJ·mol-1 for Pb6B11O18(OH)9 were obtained on the basis of the appropriate thermochemical cycles.The research results of this thesis enrich the contents of borate chemistry, and provide physic-chemical data for the application of some lead borates, which are of significance to the exploitation of the lead borates NLO material.