Scientists from St Petersburg University obtain novel red-emitting compounds for lighting devices

Inorganic luminescent compounds — phosphors — are widely used in LED lamps, which operate with virtually no heat and are smaller and brighter than incandescent light bulbs. LEDs are also more economical in service, more flexible in application and have a longer life. For commercial applications, phosphors should be resistant to various chemicals and temperature fluctuations, have high luminescence efficiency, long life and be easy to manufacture.

The results of the research supported by a grant from the Russian Science Foundation are published in the scholarly journal Solid State Sciences.

Currently, borates — salts of boric acid — are considered a promising basis for the production of inorganic phosphors. These compounds are chemically and thermally stable, and relatively low temperatures (600-1,000°C) are sufficient for their synthesis. Boron atoms can form different geometries of inorganic complexes—linear, triangular, and tetrahedral — which makes it possible to obtain a range of different crystal structures.

The scientists from St Petersburg University and the Grebenshchikov Institute of Silicate Chemistry have synthesised mixed borates of strontium, bismuth and europium with different ratios of bismuth and europium atoms in the molecule. Powders of strontium carbonate, boric acid and oxides of bismuth and europium were sintered at temperatures of 650-900°C. In this process, the reaction occurred through diffusion — the mutual penetration of particles of one substance into another.

The scientists determined the structure of the borates using single-crystal X-ray diffraction, in which X-rays are refracted on a single crystal of the synthesised compound. This method made it possible to determine the mutual arrangement of the atoms in the borates and the lengths of the chemical bonds.

The researchers also analysed the optical and luminescent properties of the resulting compounds. It turned out that the brightest luminescence was produced by a compound containing three times more bismuth atoms than europium atoms. When there are more europium ions, the efficiency of the radiation decreases, which is due to a change in the crystal structure of the substance.

Additionally, the authors used luminescence spectra to determine the coordinates of the synthesised borates in the CIE colour space — a colour coding model in which each colour has specific coordinates. This model is widely used in the computer, television and industrial sectors.

"The results obtained show that the phosphors synthesised in the study are promising candidates for use in LED lamps. This is because they are relatively easy to produce and their properties are comparable to the commercial standard red phosphor currently used in various lighting devices," said Stanislav Filatov, Principal Investigator, Professor in the Department of Crystallography at St Petersburg University.

St Petersburg University, the oldest university in Russia, was founded on 28 January (8 February) 1724. This is the day when Peter the Great issued a decree establishing the University and the Russian Academy of Sciences. St Petersburg University today is a major centre for education and research. More than 20,000 students study here, and more than 15 major laboratories and 23 resource centres have been established as part of the country’s leading Research Park. Graduates of the University have been recipients of the Nobel and Fields Prizes on multiple occasions.

Recently, St Petersburg, the Northern Capital of Russia, officially introduced a new holiday — Day of St Petersburg University — which has been included in the St Petersburg Law "On holidays and commemorative days in St Petersburg".

In February 2025, a ceremonial event was held, during which Roscosmos cosmonauts presented the University with the "300^th anniversary of St Petersburg University" flag, which had travelled to the International Space Station and back.