Barium green color. Barium sulfate is used as an insoluble additive to oil well drilling fluid, as well as in a purer form, as X-ray radiocontrast agents for imaging the human gastrointestinal tract. The soluble barium ion and soluble compounds are poisonous, and have been used as rodenticides. Barium is a soft, silvery-white metal, with a slight golden shade when ultrapure. The silvery-white color of barium metal rapidly vanishes upon oxidation in air yielding a dark gray oxide layer. Barium has a medium specific weight and good electrical conductivity. Ultrapure barium is very difficult to prepare, and therefore many properties of
Barium borate 1 GW/cm at 1064 nm and 500 MW/cm at 355 nm. Beta barium borate, β-BaBO, is a nonlinear optical material transparent in the range ~190–3300 nm. It can be used for spontaneous parametric down-conversion. Its Mohs hardness is also 4.5. Barium borate has strong negative uniaxial birefringence and can be phase-matched for type I ("ooe") second-harmonic generation from 409.6 to 3500 nm. The temperature sensitivity of the indices of refraction is low, leading to an unusually large (55 °C) temperature phase-matching bandwidth. Barium borate can be prepared by reaction of an aqueous solution of boric acid with barium hydroxide. The