Study of the solubility in Na–Ba–Cl–H2O, Na–Ba–H2PO2–H2O, Na–Cl–H2PO2–H2O, and Ba–Cl–H2PO2–H2O ternaries, and in Na+, Ba2+/Cl−, (H2PO2)−//H2O reciprocal quaternary system at 0°C