Langanite (La3Ga5.5Nb0.5O14, growth atmosphere: 99% Ar + 1% O2) and kanigasite (Ca3NbGa3Si2O14, growth atmosphere: 100% Ar) crystals grown by the Czochralski technique in Ir crucibles along <0001> direction have been firstly investigated by neutron diffraction. The difference between the compositions of upper (La2.935(2)□0.065)(Ga0.450Nb0.550(3))Ga3(Ga1.965(4)□0.035)(O13.90(1)□0.10) and lower (La2.940(1)□0.060)(Ga0.590Nb0.410(2))Ga5(O13.82(1)□0.18) parts of orange langanite crystal was found. It was established that the colorless Ca3NbGa3Si2O14 crystal grown by using the single-crystal charge has the composition (Ca2.95□0.05(1))NbGa3Si2O14 and is less defective in comparison to the yellow one grown by using the charge prepared by conventional solid-state reaction. For Ca3NbGa3Si2O14 and La3Ga5.5Nb0.5O14 crystals the possibility of microtwin formation (two unit cells connected by the translation: 1/2 z) was revealed for the first time. It was found that the difference between the color of crystals is attributed to the qualitative differentiation of oxygen vacancies. 1. Introduction Kanigasite (Ca3NbGa3Si2O14, CNGS) and langanite (La3Ga5.5Nb0.5O14, La3(Ga0.5Nb0.5)Ga5O14, LGN) crystals have langasite-type structure (sp.gr. P321, ) similar to langasite (La3Ga5SiO14, La3Ga4(GaSi)O14, LGS) and langatate (La3Ga5.5Ta0.5O14, La3(Ga0.5Ta0.5)Ga5O14, LGT) ones. Langasite family crystals possess a number of properties that ensure the successful application in piezo-, opto-, and acoustoelectronics. These properties include the absence of structural phase transitions up to the melting temperature, high thermal stability, high values of electromechanical coupling coefficients and acoustic , and low loss propagation of the elastic waves in the crystal. Langanite is characterized by the largest values of piezoelectric modules ( C/N; C/N) [1] among the langasite family crystals, whereas the CNGS has high values of the acoustic quality factor ( ) due to the fact that every atom in its structure is located in an individual position. The crystallographic orientation with a zero TCF (temperature coefficient of frequency) at room temperature was found for this crystal. The traditional Czochralski method is the primary method of obtaining large-size LGN [2–7] and CNGS [6–16] single crystals up to 200？mm in diameter. However, the use of these compounds depends on the composition (in other words, on the type and concentration of point defects), which does not coincide with the composition of the initial charge. There are quite a few works devoted to the structural investigation
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