Phase diagram of the Tl3PbBr5-Tl3PbBr5 section of the reciprocal system 2TlI + PbBr2 double left right arrow 2TlBr + PbI2 was investigated by differential thermal and X-ray phase analysis methods on 18 alloys. The section is quasi-binary, of eutectic type, with the invariant point coordinates 76 mol% Tl3PbBr5 and 355 degrees C. The addition of Tl3PbI5 results in stabilization of the high-temperature (HT) modification of Tl3PbBr5 forming a substitution solid solution, which at room temperature occupies the concentration range of similar to 9-71 mol% Tl3PbBr5. The solid solution range of low temperature (LT) Tl3PbBr5 is minor (0-similar to 6 mol% Tl3PbBr5), and that of Tl3PbI5 is in the range of similar to 71-100 mol% Tl3PbBr5. Tl3PbBr2.5I2.5 crystals were grown by Bridgman-Stockbarger method; they belong to wide-gap semiconductors with E-g decreasing from 2.45 eV at 100 K to 2.36 eV at 300 K. Based on X-ray photoelectron (XP) core-level spectra, one can conclude that the Tl3PbBr2.5I2 5 crystal reveals the XP fine-structure peculiarities well ascribed to thallium, lead, bromine and iodine, constituent atoms of the quaternary halide under consideration. Treatment of the Tl3PbBr2.5I2 5 crystal with 3 keV Ar+ ions reveals that the Pb-I bonds are somewhat weaker in this compound, as compared to the Pb-Br, TI-Br and Tl-I bonds. Comparison with the similar XP measurements of the related ternary bromide and iodide (Tl3PbBr5 and Tl3PbI5) indicates that the positive effective charge of lead atoms gradually decreases when going from Tl3PbBr5 to Tl3PbBr2.5I2.5 and, further, to Tl3PbI5, while the charge state of thallium atoms does not change in the above sequence of compounds. In the sequence Tl3PbEr5 -> Tl3PbBr2.5I2.5 -> Tl3PbI5 the relative peak intensity of the XP valence-band spectrum monotonously decreases. The laser stimulated piezooptics at 1150 nm excited by 1540 nm Er:glass nanosecond laser and its second harmonic generation are reported.