Precise X-ray powder diffraction patterns of hydrated and anhydrous thallium pentaborates have been collected on a D5000 diffractometer with a primary monochromated beam (λ Cu Kα1=1.5406 Å). Refinement of indexed reflections led to the following unit cell parameters: a=11.279(1)Å, b=7.1507(6)Å, c=13.953(1)Å, β=94.164(7)° in the P21/c space group with Z=4, Dx=2.713 gcm−3, Dm=2.6 gcm−3 for Tl[B5O6(OH)4]·2H2O and a=7.5698(5)Å, b=11.9509(6)Å, c=14.759(1)Å in the Pbca space group with Z=8, Dx=3.844 gcm−3, Dm=3.6 gcm−3 for TlB5O8. Very good Smith and Snyder figures of merit have been obtained: F30=139.7 (0.0043, 50) for Tl[B5O6(OH)4]·2H2O and F30=139.3 (0.0054, 40) for TlB5O8.
