Role of light lithium ion in formation of high-temperature exciton-like luminescence in NaCl:Li crystals

Abstract The mechanisms governing the formation of luminescence yield involving the isoelectronic lithium ion in NaCl:Li crystals were investigated using thermally stimulated, luminescence and absorption spectroscopy, along with the temperature dependence of vacancy jump frequency. It was established that the presence of the light lithium ion (Li + ) in NaCl:Li crystals enhances the efficiency of radiation defect formation compared to pure NaCl crystals. This effect manifests itself in the appearance of high-temperature thermally stimulated luminescence peaks in the temperature region of 570 K, as well as in the absorption spectra of polyhalide complexes ((Cl 2 ) 2 ) with a maximum at 5.8 eV. High-temperature TSL peaks (395 K and 570 K) with a maximum at 3.4 eV were newly observed in NaCl:Li crystals, coinciding with the spectra of x-ray luminescence (XRL) and tunneling luminescence (TL). The spectral identity of TSL, XRL, and TL in NaCl:Li crystals, characterized by a single luminescence band with a maximum at 3.4 eV, allows its nature to be interpreted as the radiative relaxation of an exciton-like formation localized in the field of the light Li + impurity ion. Calculations of the temperature dependence of anion ν ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mrow> <mml:mi>υ</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>a</mml:mtext> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) and cation ν ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mrow> <mml:mi>υ</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> ) vacancy jump frequencies indicate that the thermal dissociation of radiation-induced polyhalide defects ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mtext>Cl</mml:mtext> </mml:mrow> <mml:mrow> <mml:mtext>3</mml:mtext> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext> – </mml:mtext> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> and (Cl 2 ) 2 ) in NaCl:Li crystals predominantly occurs due to the mobility of cation vacancies, because at 555 K, ν ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mrow> <mml:mi>υ</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>c</mml:mtext> </mml:mrow> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> ) = 46 × ν ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mrow> <mml:mi>υ</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>a</mml:mtext> </mml:mrow> <mml:mrow> <mml:mtext> + </mml:mtext> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ). As a result of the thermal decomposition of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mtext>Cl</mml:mtext> </mml:mrow> <mml:mrow> <mml:mtext>3</mml:mtext> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext> –</mml:mtext> </mml:mrow> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> - and (Cl 2 ) 2 -centers in NaCl:Li crystals, a flux of non-relaxed holes ( h ) is generated. Upon their recombination with electrons ( e ) in the field of the Li + ion, exciton-like luminescence <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>e</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>SL</mml:mtext> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mtext>0</mml:mtext> </mml:mrow> </mml:msubsup> <mml:msup> <mml:mrow> <mml:mtext>(Li</mml:mtext> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> is formed.