Scintillation Properties and Electronic Structures of the Intrinsic and Extrinsic Mixed Elpasolites ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_3{\mathrm{I}}_3$ ($R=\mathrm{La}$, Y)

Scintillators attract wide research interest for their distinct applications in radiation detection. Elpasolite halides are among the most promising scintillators due to their high structural symmetry and good scintillation performance. A better understanding of their underlying scintillation mechanism opens up possibilities in scintillator development. In this work, we employ a variety of experimental techniques to study the two mixed-anion elpasolites ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_3{\mathrm{I}}_3$ ($R=\mathrm{La}$, Y). The emission of intrinsic ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_3{\mathrm{I}}_3$ with a light yield ranging from 20 000 to $40000\mathrm{ph}/\mathrm{MeV}$ is dominant by self-trapped exciton emission. Partial substitution of $R$ with Ce introduces a competing emission, the ${\mathrm{Ce}}^{3+}$ $5d$-to-$4f$ radiative transition. Ab initio calculations are performed to investigate the electronic structures as well as the binding energies of polarons in ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_6$. The calculated large self-trapped exciton binding energies are consistent with the observed high light yield due to self-trapped exciton (STE) emission. The unique electronic structure of halide elpasolites as calculated enhances the STE stability and the STE emission. The highly tunable scintillation properties of mixed-anion elpasolites underscore the role of their complex scintillation mechanism. Our study provides guidance for the design of elpasolite scintillators with exceptional energy resolution and light yield desirable for applications.

Figure 1

(a) The crystal is grown in a gold-coated transparent furnace and (b) a transparent crystal boule of ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$: 2%Ce; (c),(d) the cubic crystal structure of mixed-anion elpasolite viewed from two perspectives. The drawings in (c) and (d) are produced using Visualization for Electronic and Structure Analysis (vesta) software [41].

Figure 2

RL spectra comparison of intrinsic and extrinsic samples of (a) ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ and (b) ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$. The emission peaks of the extrinsic samples are fit with a Gaussian function, as shown in the blue curves. The data are normalized by the maximum peak intensity. (c) The quantum efficiency curve of a common PMT (Hamamatsu H3177-50). The spectra of extrinsic samples ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ and ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ are adapted from Ref. [37].

Figure 3

PL excitation spectra at 40 K of (a) ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$: 5% Ce and (b) intrinsic ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$. Both spectra are normalized to the maximum peak. Excitation spectra are monitored at various emission wavelengths. The highlighted region from 250 to 280 nm indicates the strong exciton-excitation band in intrinsic samples.

Figure 4

PL emission spectra at 40 K of (a) ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$: 5% Ce and (b) intrinsic ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$. Both spectra are normalized to the maximum peak. (c) The integrated emission spectra under different excitation wavelengths of intrinsic ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ (blue dot) and extrinsic ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$: 5% Ce (red dot), respectively. The integration range is from 350 to 550 nm. The integrated intensity is calculated with the un-normalized raw data. The raw intensity comparison without normalization can be found in Supplemental Material [55]. The intrinsic sample gives strong emission intensity under the exciton excitation at a shorter wavelength.

Figure 5

PL excitation spectra at 40 K of (a) ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$: 5% Ce and (b) intrinsic ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$. Both spectra are normalized to the maximum peak.

Figure 6

PL emission spectra at 40 K of (a) ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$: 5% Ce and (b) intrinsic ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$. Both spectra are normalized to the maximum peak.

Figure 7

(a) PL excitation and (b) PL emission spectra of intrinsic ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ at RT; the RL spectrum of ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ at RT is also plotted for comparison.

Figure 8

(a) PL decay of ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$: 5% Ce at 40 K: The emission at 420 nm is monitored with excitation wavelengths of 295, 333, and 370 nm. The instrumental response is measured to be less than 1 ns, which can be ignored in the decay fitting. The PL decay curves are fit with a single decay exponential function. PL decay of intrinsic ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ at (b) 40 K and (c) RT. The emission at 420 nm is monitored with excitation wavelengths of 295 and 370 nm. The emission cannot be detected with 295-nm excitation in the 40-K measurement, because the cryogenic sample holder blocks the weak emission.

Figure 9

(a) PL decay of ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$: 2% Ce at 40 K: The emission at 420 nm is monitored with excitation wavelengths of 295, 333, and 370 nm. The PL decay curves are fit with a single decay exponential function. PL decay of intrinsic ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ at (b) 40 K and (c) RT. The emission at 420 nm is monitored with excitation wavelengths of 295 and 370 nm. The emission cannot be detected with 295-nm excitation in the 40-K measurement, because the cryogenic sample holder blocks the weak emission.

Figure 10

Scintillation decay profiles of intrinsic and extrinsic samples of (a) ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ and (b) ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$. The extrinsic decay curves are fit with three decay exponential functions, and the intrinsic decay curves are fit with two decay exponential functions. The fitting curves are shown in solid gray lines. The scintillation rising time of the extrinsic ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ and ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ can be found in Ref. [55].

Figure 11

Gamma-ray pulse height spectra of intrinsic and extrinsic samples of (a) ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ and (b) ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$. The photopeak at 662 keV is highlighted to better illustrate the position. Cs-137 source is used. The pulse height spectra of extrinsic samples ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ and ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ are adapted from Ref. [37].

Figure 12

Band structure of (a) ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_6$ and (b) ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_6$; (c) density of states (DOS) of ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_6$.

Figure 13

Energy diagram of ${\mathrm{Cs}}_2{\mathrm{NaLaBr}}_3{\mathrm{I}}_3$ (left) and ${\mathrm{Cs}}_2{\mathrm{NaYBr}}_3{\mathrm{I}}_3$ (right) at 40 K.

Figure 14

Diagram of three different scintillation decay processes in the mixed elpasolite scintillators.