## 1 Introduction  
In recent years, there has been a global tendency to replace radio communication lines within satellite constellations, as well as between satellites and ground objects [1]. There is a fundamental problem of searching for an active laser medium for use in atmospheric and space optical communication systems (FSO). To organize space or long-range atmospheric optical communication, laser systems with high energy characteristics and, therefore, laser media that can withstand extreme optical, radiational and thermal loads are needed.  
High melting rare-earth-doped sesquioxides can be promising materials for FSO applications. Due to the disordered crystal structure, rare-earth-doped mixed sesquioxides (such as yttrium scandate YScO\({}_{3}\)) have inhomogeneously broadened spectral lines [2; 3] which provides an advantage for its highly efficient pumping with modern diodes with low requirements for their thermal stabilization. Instead of the Czochralski method [4; 5], we propose the use of the laser-heated pedestal method, which is more suitable for the growth of the high-melting crystals of good optical quality and small size required for use in equipment. In this approach, crystal synthesis is accompanied by high-temperature heating with laser radiation and subsequent rapid cooling.  
Scandia (Sc\({}_{2}\)O\({}_{3}\)) and yttria (Y\({}_{2}\)O\({}_{3}\)) crystallize in a bixbyite structural type and have high thermal conductivity (two times more than yttrium aluminum garnet) [6] and a wide transparency window of 0.25 to 9.6 \(\upmu\)m [7]. These properties are critically important for solid-state laser application. [3; 8; 9] Therefore, yttria and scandia may become alternative materials and replace YAG crystals in some applications. Ceramics based on rare-earth oxides [10] and mixed sesquioxides [11; 12; 13; 14; 15; 16] are declared as excellent laser materials. Moreover, sesquioxides are advanced materials for application as white light emitting phosphors, [17; 18] high effective luminescent materials, [19; 20] rare-earth magnets, [21; 22] optical recording and storage [23].  
Yttrium scandate has a polymorphic nature. Its high-temperature modification [24; 25], similar to yttria [26] and scandia [27], belongs to a bixbyite structural type with the space group _la3_. The crystal structure includes one site of oxygen ion with local symmetry \(C_{1}\) (Wykoff position 48\(e\)) and two symmetrically independent sites of cations with local symmetry \(C_{3i}\) ([PERSON] position 8\(b\)) and \(C_{2}\) ([PERSON] position 24\(d\)). The ionic radii of yttrium (0.9 A), scandium (0.83 A) and rare-earths are close, and the substitution of basic cations by rare-earth impurities does not require charge compensation and it is crystallochemically unforbidden.  
Here we have synthesized and characterized the YScO\({}_{3}\) crystal fibers, pure and doped by 0.1 at. % Nd\({}^{3+}\) and Tm\({}^{3+}\) ions. Nd\({}^{3+}\) and Tm\({}^{3+}\) ion situate in different ends of lanthanoid group and have a maximal different ionic radius. Nd\({}^{3+}\) belongs to large rare-earth ions while Tm\({}^{3+}\) is a small ion. Moreover, Nd\({}^{3+}\) is a Kramers ion with an odd number of electrons, and Tm\({}^{3+}\) belongs to non-Kramers ions, with an even number of electrons. Therefore, Nd\({}^{3+}\) and Tm\({}^{3+}\) are maximally different ions and would presumably have different spectral behavior, which is related to the local structure of optically active centers. We have used selective laser spectroscopy methods, in which the luminescence spectra are recorded at selective excitation and, conversely, the excitation spectra are measured at selective detection. This approach allows us to trace local changes at included ion-activators. In this work, we have focused on studying the influence of rare-earth ion radius size on spectroscopic properties and the character of rare-earth impurities' distribution.