| Summary: | Guanidinium tetraborate anhydrate is formed as a new phase of guanidinium borate by heating the precipitate from the mixed solution of guanidinium carbonate and boric acid at 115–250°C. The new anhydrate phase is also formed by solid-state reactions of the powder mixture of guanidinium carbonate and boric acid at 115°C, and this route is advantageous over the liquid-phase method for industrial production. The structural and thermal properties of the new anhydrate phase are investigated using XRD and DTA-TG in comparison with the reported phase of guanidinium tetraborate dihydrate, [C(NH2)3]2[B4O5(OH)4]･2H2O. The chemical formula of the new phase is estimated to be [C(NH2)3]2[B4O5(OH)4]. This new anhydrate phase belongs to the monoclinic system, and the lattice constants are estimated at a = 0.7262 (9), b = 0.7868 (14), c = 0.5101 (9) nm, β = 87.61 (13)°, V = 0.2912 nm3, Z = 1 for the liquid-phase product, and a = 0.7250 (11), b = 0.7906 (21), c = 0.5083 (11) nm, β = 87.62 (14)°, V = 0.2911 nm3, Z = 1 for the solid-state reaction product.
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