It is shown that introducing small amounts of P 2 O 5 , which do not give rise to opacification and opalescence, into silicate glasses strongly affects the dilatometric properties of glasses. This gives an idea of the structural changes and could be of practical value.The introduction of small amounts of phosphorus oxide, which do not give rise to opacification and opalescence, into silicate glasses can be used to improve the technological and functional properties. For example, is shown in [1, 2] that adding 0.3% 2 P 2 O 5 to the commercial compositions of sheet glass from the vertical drawing system decreases the liquidus temperature by 25 -50°C and decreases the linear rate of growth of crystals, which makes it possible to increase the drawing rate and improve the quality of the product.The effect of small additions of P 2 O 5 on the properties of container glass with a lower content of magnesium oxide (1.5 -1.8%) has been investigated in [3]. It was established that replacing 1% SiO 2 by P 2 O 5 does not greatly change the high-temperature viscosity, on the basis of which it is asserted that the partial coefficients of P 2 O 5 for these properties are equal to the coefficients for SiO 2 . The technological advantage of introducing small amounts of P 2 O 5 is a decrease of the liquidus temperature (by 15°C), which makes it possible to decrease the founding temperature and to increase the content of the cheaper component (CaO) in the composition. For chemical stability, it is noted that the mass losses in water decrease. The authors attribute this to an increase in the polymerization of the structure. On the whole, the effect of phosphorus oxide on the basic physical-chemical properties of glasses is not unique. It is determined by the concentration of P 2 O 5 and the chemical composition of the matrix glass, primarily by the presence of alkali oxides and Al 2 O 3 in the glass [4,5].Silicophosphate glasses are systems with two glassforming cations, each of which can form its own characteristic anionic motif. Phosphorus is a stronger cation than silicon (according to the data in [6], the ionic radii of Si and P are 0.039 and 0.035 nm, the energy of the Si -O bond is 443 kJ/mole and the energy of the P -O is 464 kJ/mole, the coordination number of Si and P is 4). For phosphorus, there are stoichiometric differences with silicon (a higher charge of the ion and the presence of the double bond P = O), which could be the reason for their structural incompatibility.In the opinion of the authors of [7], the introduction of small quantities of P 2 O 5 into glassy silica results in a distortion and rupture of the bonds and increases the content of non-bridge oxygen (Fig. 1a ). For substantial P 2 O 5 content (about 44%, molar content), the formation of a single silicon -phosphate framework where the tetrahedra [P 5+ O 4 ] have two P -O bonds, equal in length and multiplicity, can form. For high concentrations of phosphorus, judging from the character of the change in the properties of the glasses (density, thermal expans...