Thermodynamic properties of two mixed alkali metal borates with NLO behaviour: Li6Rb5B11O22 and Li4Cs3B7O14

• Li6Rb5B11O22 and Li4Cs3B7O14 have been synthesized and characterized.
• The enthalpies of solution of title two borates were measured.
• The enthalpies of solution of LiCl·H2O(s) were measured.
• ΔfHmo for title two borates were obtained by the thermochemical cycles.

Two pure mixed alkali metal borates with Non-Linear Optical (NLO) properties, Li6Rb5B11O22 and Li4Cs3B7O14, have been synthesized by a high-temperature solid state reaction, and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Li6Rb5B11O22 and Li4Cs3B7O14 in 1 mol · dm−3 HCl (aq), and of LiCl·H2O(s) in (1 mol · dm−3 HCl + H3BO3 + RbCl/CsCl) (aq) have been determined by microcalorimeter at T = 298.15 K, respectively. From these data and with the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1 mol · dm−3 HCl (aq), together with the use of the standard molar enthalpies of formation for LiCl·H2O(s), RbCl(s)/CsCl(s), H3BO3(s), HCl(aq) and H2O(l), the standard molar enthalpies of formation of −(11173.1 ± 9.5) kJ · mol−1 for Li6Rb5B11O22 and −(7145.1 ± 5.9) kJ · mol−1 for Li4Cs3B7O14 were obtained on the basis of the appropriate thermochemical cycles.