This paper reports on the development of a technology involving 100 Mo-enriched scintillating bolometers, compatible with the goals of CUPID, a proposed nextgeneration bolometric experiment to search for neutrinoless double-beta decay. Large mass (∼ 1 kg), high optical quality, radiopure 100 Mo-containing zinc and lithium molybdate crystals have been produced and used to develop high performance single detector modules based on 0.2-0.4 kg scintillating bolometers. In particular, the energy resolution of the lithium molybdate detectors near the Q-value of the doublebeta transition of 100 Mo (3034 keV) is 4-6 keV FWHM. The rejection of the α-induced dominant background above 2.6 MeV is better than 8σ . Less than 10 µBq/kg activity of 232 Th ( 228 Th) and 226 Ra in the crystals is ensured by boule recrystallization. The potential of 100 Mo-enriched scintillating bolometers to perform high sensitivity double-beta decay searches has been demonstrated with only 10 kg×d exposure: the two neutrino double-beta decay half-life of 100 Mo has been measured with the up-to-date highest accuracy as T 1/2 = [6.90 ± 0.15(stat.) ± 0.37(syst.)] × 10 18 years. Both crystallization and detector technologies favor lithium molybdate, which has been selected for the ongoing construction of the CUPID-0/Mo demonstrator, containing several kg of 100 Mo.

Large lithium molybdate (Li 2 MoO 4 ) crystal boules were produced by using the low thermal gradient Czochralski growth technique from deeply purified molybdenum. A small sample from one of the boules was preliminary characterized in terms of Xray-induced and thermally-excited luminescence. A large cylindrical crystalline element (with a size of ⊘40 × 40 mm) was used to fabricate a scintillating bolometer, which was operated aboveground at ∼ 15 mK by using a pulse-tube cryostat housing a highpower dilution refrigerator. The excellent detector performance in terms of energy resolution and α background suppression along with preliminary positive indications on the radiopurity of this material show the potentiality of Li 2 MoO 4 scintillating bolometers for low-counting experiment to search for neutrinoless double beta decay of 100 Mo.

Crystals of the Mn Ba(BO)F phase were grown from a high-temperature solution. This new fluoride borate is built of positively charged [Ba(BO)] blocks, the so-called "anti-zeolite" pattern. Using X-ray single-crystal diffraction, the bulk atomic arrangement in the centrosymmetric tetragonal unit cell in I4/ mcm could be elucidated. Crystals of the (MnF) group-containing solid solution Mn Ba(BO)F are dark brown in color in contrast to the differently colored crystals of (LiF) group-containing "anti-zeolite" LiBa(BO)F ( P4 bc). According to the electron spin resonance and optical spectroscopic investigation, the absorption spectrum of LiBa(BO)F crystals results from the absorption of light by both exciton and free charge carriers and can be tuned by varying the initial composition of the high-temperature solution.

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A comprehensive study of the BaF2–Ba3(BO3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba7(BO3)4−yF2+3y. The latter exhibited (BO3)3−↔ 3F− anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4−yF2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3−↔ 3F− isomorphism, allows for the disclosure of the mechanism of (BO3)3−↔ 3F− heterovalent anionic substitution in fluoride borates via [(BO3)F]4− tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3−↔ 3F− substitution.

Detailed study of the BaB 2 O 4 -BaF 2 -BaO system resulted in the discovery of the new Ba 7 (BO 3 ) 42x F 2+3x solid solution belonging to the BaF 2 -Ba 3 (BO 3 ) 2 section. The distinguishing feature of the crystal structure of Ba 7 (BO 3 ) 42x F 2+3x phase is its extensive (BO 3 ) 32 « 3F 2 anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba 7 (BO 3 ) 3.51 F 3.47 (x = 0.49) single crystals (space group P6 3 ; a = 11.18241(11) A ˚, c = 7.23720(8) A ˚). The area of homogeneity for Ba 7 (BO 3 ) 42x F 2+3x solid solution spans between Ba 7 (BO 3 ) 3.35 F 3.95 and Ba 7 (BO 3 ) 3.79 F 2.63 compositions (0.21 , x , 0.65). Also, a new orthorhombic phase with a tentative composition of Ba 5 (BO 3 ) 3 F has been identified in XRD powder patterns and indexed with cell parameters a = 7.605 A ˚, b = 14.843 A ånd c = 10.291 A ˚.

The phase formation in the BaB2O4- NaF system has been studied by the methods of solid-state synthesis, visual polythermal analysis, spontaneous crystallization on platinum loop and differential thermal analysis. A new version of BaB2O4−NaF phase diagram has been obtained as the result of comprehensive investigations. Conditions for primary crystallization of the new compound Ba2Na3(B3O6)2F (hexagonal system, P63/m, a = 7,346(1) Å, c = 12,637(2) Å) have been determined. Bulk crystals of Ba2Na3(B3O6)2F have been grown. An absorption edge in the UV spectrum of the above compound has been characterized.