Lanthanum gallium silicate crystal (langasite - LGS) was first grown in Russia at the beginning of the 1980?s, as a result of joint development of the Moscow State University (MGU) and the Institute of Crystallography of the Russian Academy of Sciences. Initially this crystal was intended for use in laser engineering but for a number of reasons it did not find a suitable application in this area. The profound research of the physical properties of langasite carried out by the Institute of Crystallography revealed its piezoelectric characteristics, which predetermined the use of langasite in piezoelectrical engineering.
The sphere of piezoelectrical engineering embraces radio frequency control elements (resonators) and selection devices (filters). The predominant material used for this purpose is quartz, which possesses highly stable frequency and temperature characteristics. However the use of quartz in piezoelectric filters is quite limited because its index of electromechanical coupling K²el.mech is very low and averages about 7%. This quality confines the use of quartz to narrow band filters with the relative value of the pass band to the centre frequency up to 0.3%.
Present-day digital methods of signal treatment require a broader pass band in intermediate frequency amplifiers. To produce such filters with quartz it is necessary to install additional elements (e.g. inductive coils), which increases the size and weight of the gadget. The alternative solution is the use of a piezoelectric material with a higher K2el.mech. value.
A suitable mateial for broad band filters is lithium tantalate. The relative pass band of the filters based on lithium tantalate lies in the range of 0.6 ? 4 %. However the frequency-temperature stability and the quality (Q-factor) of such filters remain quite low, hampering their unanimous promotion. An ideal crystal for a filter with a relative pass band 0.3-0.8% should have properties intermediate between those of quartz and lithium tantalate. The best solution to this problem is langasite. Langasite-based filters possess relative pass band between 0.3-0.85%. Besides, langasite is a perfect material for resonators with 0.8% resonance required in new types of oscillators.
The most promising application of langasite crystal is piezoelectric substrates for filters operating both at bulk waves (BAW) and surface waves (SAW).
The basic piezoelectric properties of different crystals operating at bulk waves are represented and compared in Chart 1. The chart shows that all the main characteristics of langasite, such as index of electromechanical coupling, frequency, Q-factor and temperature-frequency coefficient are intermediate between those of quartz and lithium tantalate.