Optical Thermometry Based on Vibration Sidebands in Y2MgTiO6

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Optical Thermometry Based on Vibration Sidebands in Y2MgTiO6:Mn4+ Double Perovskite Peiqing Cai,† Lin Qin,† Cuili Chen,† Jing Wang,† Shala Bi,† Sun Il Kim,† Yanlin Huang,‡ and Hyo Jin Seo*,† †

Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea ‡ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ABSTRACT: Mn4+-doped Y2MgTiO6 phosphors are synthesized by the traditional solid-state method. Powder X-ray diffraction, scanning electron microscope, and energy-dispersive X-ray spectrometer are employed to characterize the samples. The Mn4+-doped Y2MgTiO6 phosphors show the far-red emission at ∼715 nm, which is assigned to the 2Eg → 4A2 spin-forbidden transition of Mn4+. The temperaturedependent luminescent dynamics of Mn4+ is described by a complete model associated with electron−lattice interaction and spin−orbit coupling. The noncontact optical thermometry of Y2MgTiO6:Mn4+ is discussed based on the fluorescence intensity ratio of thermally coupled anti-Stokes and Stokes sidebands of the efficient ∼715 nm far-red emission in the temperature range of 10−513 K. The maximum sensor sensitivity of Y2MgTiO6:Mn4+ is determined to be as high as 0.001 42 K−1 at 153 K, which demonstrates potential applications for the optical thermometry at low-temperature environments.

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INTRODUCTION Recently, the noncontact optical thermometry based on the fluorescence intensity ratio (FIR) has been a matter of interest due to the small size, good biological compatibility, high sensitivity, and wide range of the temperature response in different application areas.1−4 Until now, the candidates of the universal optical sensing are the phosphors doped with lanthanides, such as Tm3+, Pr3+, Ho3+, and Er3+,5−8 in which the FIR is obtained by the peak emission from a pair of thermally coupled states in up-conversion processes. However, in the up-conversion process in the rare-earth doped phosphors, the IR laser diode with high power is usually necessary as a pumping source, because the luminescence quantum efficiency (QE) of the up-conversion emission is low.9 As a result, the application of the lanthanide-doped optical thermometry is restricted by the complicated experimental setups of the IR system to characterize the optical temperature behaviors of the samples and the heavy use of the costly rareearth. Hence, it is necessary to blaze new trails in discovering the optical thermometry with low-cost, energy conservation, and high sensitivity. Currently, to overcome the weakness of the red range in commercial white light-emitting diodes (LEDs; strategy: InGaN blue chips combining with yellow YAG: Ce 3+ phosphors),10 several kinds of novel Mn4+-doped fluoride phosphors have been developed, such as A2XF6 and BXF6, where A = K, Na, Cs, and Rb, B = Ba and Zn, and X = Ti, Si, Ge, and Zr.11−17 These Mn4+-doped fluoride phosphors exhibit © XXXX American Chemical Society

the effective red emission in the range of 600−650 nm under near-UV and blue LED excitations. However, the Mn4+activated fluoride phosphors are unstable at high temperature and harmful to the environment during the material preparation due to the heavy use of HF,18−21 which restricts the practical application of these phosphors. In contrast, the Mn4+-activated oxide compounds are environmentally friendly during the preparation procedure. Currently, several Mn4+activated double perovskites have been discovered and applied to plant cultivation and illumination, such as Mn4+-doped Ba2LaNbO6,22 La2MgTiO6,23 Gd2ZnTiO6,24 and Gd2MgTiO6.25 Nevertheless, the significant thermal quenching property and near-infrared emission (>650 nm) of Mn4+doped double perovskites indicate that this kind of material is not suitable for use in LED phosphors but can be employed as optical thermometry sensors.26 The A2BB′O6-type double perovskite is flexible and adjustable by crystal structure engineering. The luminescence properties of luminescent ions are strongly influenced by the surrounding environmental factors such as B site cation ordering, octahedral distorting and tilting, etc. The sensor sensitivity of FIR-based optical thermometry can be easily adjustable by changing the crystal field of the double perovskite. More recently, the Mn4+activated oxyfluoride Cs2WO2F427 was introduced as an optical thermometry, but the synthetic route is also harmful, and the Received: November 20, 2017

A

DOI: 10.1021/acs.inorgchem.7b02938 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry sample is toxic. To investigate the materials with outstanding sensing features, the novel material exploitation is still going on. In this work, a comprehensive research is made on the Mn4+doped Y2MgTiO6 synthesized by conventional solid-state reaction at 1400 °C. The samples are characterized by X-ray diffraction (XRD) and refined by the Rietveld refinement method. The surface properties are studied in detail. The luminescence properties are investigated by excitation and emission spectra and decay curves at various temperatures. The luminescence characteristics are discussed on the basis of crystal field analysis. Finally, the Mn4+-doped Y2MgTiO6 phosphor as an optical thermometry based on the FIR technique is discussed to evaluate the temperature-sensing properties of the perovskite-type ceramics.

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EXPERIMENTAL SECTION

Preparations. A series of Y2MgTO6:x%Mn4+ (x = 0−1) phosphors were synthesized by the conventional high-temperature solid-state reaction method. The starting materials Y2O3 (99.9%), MgO (99.9%), TiO2 (99.9%), and MnO2 (99.9%) were weighed according to the stoichiometric ratio with 2 wt % excess H2BO3 as a flux. The mixed powders were thoroughly ground with a small quantity of ethanol in an agate mortar for 20 min, prefired at 1000 °C for 8 h, reground for 20 min, pressed to a certain thickness with a manual pill tablet press, and sintered at 1400 °C for 24 h in ambient atmosphere. After the furnace cooled to room temperature, the products were fully ground again to obtain the resulting phosphor powders. Characterization. The phase of the Y2MgTiO6:Mn4+ powders was identified by using an X-ray Rigaku D/Max diffractometer operating at 40 kV and 30 mA and equipped by Cu Kα radiation (λ = 1.5405 Å). The Rietveld refinement of the XRD data was performed with the General Structure Analysis System (GSAS) software.28 The scanning rate used for the Rietveld analysis was 8 s per step with a step size of 0.02°. The microstructures and elementary compositions were observed by a scanning electron microscope (SEM) FEI Quanta 400 attached at an S-2700 energy-dispersive X-ray spectrometer (EDS). Diffuse reflection spectra were taken on a Cary 5000 UV−Vis−NIR spectrophotometer with BaSO4 powders as a standard reference. The samples were cooled in a closed-cycle helium cryostat in the temperature range of 7−300 K. A homemade temperature control system was introduced to take measurement at temperatures above 300 K. Excitation and emission spectra were recorded using a 450 W Xe lamp dispersed by the 25 cm monochromator (Acton Research Corp. Pro-250). For decay measurements, the luminescence signal was digitized and saved by means of a 500 MHz Tektronix DPO 3054 oscilloscope. The excitation source for luminescence decays was a 355 nm pulsed laser with a pulse width of 5 ns and a repetition rate of 10 Hz (Spectron Laser Sys. SL802G) in which the luminescence was dispersed by a 75 cm monochromator (Acton Research Corp. Pro750) and observed with a photomultiplier tube (PMT; Hamamatsu R928).

Figure 1. (a) The Rietveld refinement fit of the XRD patterns of Y2MgTiO6:0.2%Mn4+ by using the GSAS program. (b) Crystal structure of the Y2MgTiO6:0.2%Mn4+ double perovskite.

Table 1. Refined Atomic Coordinate Parameters Data of Y2MgTiO6:0.2%Mn4+ at Room Temperature space group: P21/n (No. 14): a = 5.292 Å, b = 5.584 Å, c = 7.614 Å, β = 90.26°

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atom

site

Y Mg Ti O1 O2 O3

4e 2c 2d 4e 4e 4e

x

y

z

occupancy

0.5148 0.5664 0.2486 1 0.0000 0.5000 0.0000 1 0.5000 0.0000 0.0000 1 0.1838 0.2036 −0.0610 1 0.3009 0.6924 −0.0531 1 0.3891 −0.0369 0.2553 1 Rwp = 11.44%, Rp = 8.96, χ2=1.694, vol = 225.027 Å3

Uiso (Å2) 0.0042(7) 0.0021(1) 0.0023(1) 0.0038(2) 0.0012(6) 0.0016(9)

disorder usually occurs when the transition-metal cations occupy the Wyckoff position 2c (B2+ site) and 2d (B′4+ site) interchangeably and influence significantly the ferromagnetic property of the A2BB′X6 double perovskite.31−33 Although perfect order is seldom achieved in as-prepared samples, the charge exchange interaction might be barely effective between Mg2+ and Ti4+ in nearly ordered rock-salt type Y2MgTiO6 double perovskite. The superlattice diffraction peaks at 19.70° and 20.45° also indicate a rock-salt type cation ordering of Mg2+ and Ti4+.29,34 The deeper investigation of the existence of antisite disorder in B/B′ sites could be done by neutron diffraction method in further work. Figure 1b displays the schematic illustration of the monoclinic Y2MgTiO6 structure. The Ti4+ and Mg2+ ions form the alternative arrangement of TiO6 and MgO6 octahedra in two interleaved face-centered cubic (fcc) sublattices. The Y3+

RESULTS AND DISCUSSION Phase Formation and Structure Characteristics. The representative 0.2%Mn4+-doped Y2MgTiO6 sample was verified by the powder X-ray diffraction (XRD), and the XRD data were analyzed by the Rietveld refinement technique as shown in Figure 1a. Because of the lack of the structural information on Y2MgTiO6 in the crystallographic database, the reported crystallographic data of Dy2MgTiO6 with analogous structure29 was adopted as the initial structure model. Table 1 lists the refined structural parameters of Y2MgTiO6. The XRD patterns of Y2MgTiO6:0.2%Mn4+ are indexed to the crystal structure with a monoclinic unit cell with cell parameters a = 5.292 Å, b = 5.584 Å, c = 7.614 Å, and β = 90.26°, indicating a double perovskite structure with P21/n space group.30 The antisite B


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Inorganic Chemistry

Figure 2. (a) EDS spectrum of Y2MgTiO6:0.2%Mn4+ double perovskite. Elemental mapping of (b) Y, (c) Mg, (d) Ti, (e) O, and (f) integration of (b−e). (g) SEM image of Y2MgTiO6:0.2%Mn4+, (h) high-resolution XPS spectrum of Mn4+ in Y2MgTiO6:1%Mn4+.

of the Mn4+ ions in the host, the XPS spectrum for the valence state of manganese in Y2MgTiO6 was recorded. As shown in Figure 2h, the two weak peaks are assigned to 2P1/2 and 2P3/2 of the Mn4+ ions,37 indicating the presence of Mn4+ ions in the host. Luminescence Properties of Y2MgTiO6:Mn4+. The diffuse reflection spectra (DRS) of the Y2MgTiO6:x%Mn4+ (x = 0−0.8) phosphors are shown in Figure 3. Three relevant absorption bands are observed in the spectra. Two obvious stronger bands due to the spin-allowed transitions peaking at ∼350 and 500 nm correspond to the 4A2 → 4T1 transition and

ions occupy the cavities constructed by the tilted TiO6 and MgO6 octahedra, which are linked through the corners. In the Y2MgTiO6 host matrix, the Mn4+ ions may not substitute for the Y3+ ions due to the large difference in ionic radii between Y3+ (1.08 Å) and Mn4+ (0.53 Å). However, there is an opportunity for the Mn4+ ions to occupy simultaneously the Ti4+ and Mg2+ sites. In some hosts such as MgO35 and SrTiO3,36 the Mn4+ ions could perform as luminescent centers. Zhou et al.23 calculated the formation energy to estimate the possibility of the Mn4+ ions to enter the Ti4+ or Mg2+ sites. The results show that the formation energy of the Mn−Ti mutual replacement is lower than the substitution of Mg2+ by Mn4+. Moreover, considering the identical ionic valence (4+) and the closer ionic radius of Ti4+ (0.604 Å for Ti4+ and 0.72 Å for Mg2+), it is reasonable that the Mn4+ (r = 0.53 Å) ion is more stable in the Ti4+ site. The EDS scanning spectra in Figure 2a−f displays the element compositions of the as-prepared Y2MgTiO6:0.2%Mn4+. The different color spots over the dark background represent the uniform distribution of the relative elements on the external surface of the particles. The result in the element analysis shows that the atomic ratios of Y, Mg, Ti, and O are 21.2, 8.40, 11.2, and 59.2%, respectively, which corresponds approximately to the stoichiometric content of Y2MgTiO6:0.2%Mn4+. Figure 2g shows a typical granular morphology of Y2MgTiO6 with the size distribution in the range of 4−8 μm. The content of the Mn element cannot be detected by the EDS analysis due to the low doping level (∼1%). Generally, the content of Mn element should be consistent with the stoichiometric ratio of the initial weighing process, due to the Mn element is not easy to volatilize at the sintering temperature. To confirm the existence

Figure 3. DRS of Y2MgTiO6:x%Mn4+ (x = 0, 0.05, 0.2, 0.8) samples at room temperature. C


Article

Inorganic Chemistry the 4A2 → 4T2 transition of Mn4+, respectively. Another weak distinguishable band located at ∼420 nm is assigned to the spin-forbidden Mn4+: 4A2 → 2T2 transition.38 In addition, the band of the Mn4+: 4A2 → 4T1 transition overlaps partially with the band of the Mn4+−O2− charge transfer (CT) transition in the range of 300−400 nm. Figure 4a,b shows the room-

Mn4+ concentration. All the specific bands of the emission spectra in the range of 620−740 nm consist of several peaks with the maxima at 698 nm. The emission peaks are assigned to the vibronic sidebands and zero phonon line (ZPL) at 676 nm,27 originating from the nature of the parity-forbidden d−d transitions.41 Theoretically, the pure electronic 2Eg → 4A2 transition is both spin- and parity-forbidden; however, the 2Eg → 4A2 electric dipole transition can take place when the odd parity 4T2u and 4T1u states mix the even-parity d wave function of the 2Eg state.42 As a result, the selection rules are broken, and effective emission intensities are obtained from the 2Eg state (ZPL) and the phonon-associated sidebands with energies shifted from the ZPL to both high energy side (anti-Stokes sideband) and low-energy side (Stokes sideband) by the energy values of the relevant phonons. The emission intensity is significantly influenced by the Mn4+ concentration. No peak shift is observed with increasing Mn4+ concentration indicating the effect of the Mn4+ concentration on the crystal structure could be neglected. According to Figure 5a, the emission

Figure 5. (a) Integrated emission intensity of Y2MgTiO6:Mn4+ as a function of Mn4+ concentration. (b) Relation of log(I/xMn4+) and log(xMn4+) for the 2Eg → 4A2 transition of Mn4+.

intensity increases and then quenches beyond 0.2% with increasing Mn4+ concentration. The energy-transfer mechanism of the concentration quenching belongs to the exchange interaction or multipolar−multipolar interaction.37 A rough critical distance (Rc) is introduced to estimate the energytransfer process between two nearest Mn4+ ions. The equation is given by Blasse:43 Rc ≈ 2(3V/4πxcN)1/3, where V is the volume of the unit cell, xc is the critical concentration of activator ions, and N is the number of lattice sites in the unit cell. Referring to the structural data obtained by the refinement V = 225.02 Å3, N = 2, and xc = 0.002, Rc is calculated to be 47.53 Å. The calculated value is longer than 5 Å; as a result, the electric multipolar−multipolar interaction should be suitable for the energy-transfer mechanism of the concentration quenching process. In addition, according to the Dexter’s theory,44 the multipolar interaction between Mn4+ can be reflected by the following equation:

Figure 4. (a) Room-temperature excitation spectra of Y2MgTiO6:x% Mn4+ (x = 0−1) obtained by monitoring the 698 nm emission of the 2 Eg → 4A2 transition. (b) Room-temperature emission spectra of Y2MgTiO6:x%Mn4+ (x = 0−1) under the 365 nm excitation.

temperature excitation and emission spectra of the Y2MgTiO6:Mn4+, respectively. The excitation spectra exhibit the broad band absorption in the near-UV and blue-green spectral regions from 250 to 600 nm, which is well-consistent to the DRS spectra. An obvious red shift could be identified in the range of CT band with increasing Mn4+ concentration, which is similar to that observed for Gd2ZnTiO6:Mn4+.24 Moreover, the two dips located at 477.7 and 489.3 nm in the excitation band of the 4A2 → 4T2 transition should be noticed. The shapes of these dips seem to be caused by the reduction of peak absorption suggested as “Fano-Antiresonances”. This could be interpreted in terms of an interaction of the 2E and 2 T1 states with the vibrationally broadened 4T2 state.39,40 Figure 4b shows the emission spectra of the spin-forbidden 2 Eg → 4A2 transition of Y2MgTiO6:Mn4+ as functions of the

I /x = K[1 + β(x)θ /3 ]−1

(1)

where x is the activator concentration, K and β are constants, I represents the emission intensity, θ is an electric multipole index with θ = 6, 8, and 10 corresponding to the dipole−dipole D


Article

Inorganic Chemistry Dq

(d-d), dipole−quadrupole (d-q), and quadrupole−quadrupole (q-q) interactions, respectively. In Figure 5b, the relationship between log(x) and log(I/x) appears to be fitted well as linear with a slope of −(θ/3) ≈ −1.21, and the value of θ is nearly 6, indicating that the quenching mechanism is due to d-d interactions among Mn4+ ions. The spectroscopic properties of the Mn4+ ions depend strongly on the surroundings of Mn4+ in an octahedral environment in solids. The well-known Tanabe−Sugano diagram (Figure 6a) could be used to explain the splitting of

B

=

15(x − 8) (x 2 − 10x)

(3)

where parameter x is defined as x=

E(4A 2 − 4 T1) − E(4A 2 − 4 T2) Dq

(4)

From the peak energy of the 2Eg → 4A2 transition derived from the emission spectra, the Racah parameter C can be calculated by the expression E(2 E g − 4A 2)/B = 3.05C /B + 7.9 − 1.8B /Dq

(5)

According the above equations, the crystal field parameters of Dq, B, and C are calculated to be 2070, 724, and 3123 cm−1, respectively. As shown in Figure 6a, the 2E and 4T2 levels have a crossover point at Dq/B ≈ 2.3 in the energy-level diagram; above or below this value means the 3d3 transition-metal ions are in weak crystal field or strong crystal field, respectively.46 In our case, the value of Dq/B is calculated to be ∼2.9, indicating the Mn4+ ions in the Y2MgTiO6 host matrix is located at the strong crystal field. The value (2.9) of Dq/B is comparable to those in fluoride Na2SiF6:Mn4+ (Dq/B ≈ 2.8)13 and perovskite LaAlO3:Mn4+ (Dq/B ≈ 3.0).47 The luminescence decay curves as functions of temperature are presented in Figure 7a. The decays are slightly nonexponential and become shorter with increasing temperature. The total kinetic processes could be characterized by the average decay time (τ), which can be calculated as

τavg =

∫ tI(t )dt ∫ I(t )dt

(6)

where I is the intensity at time t, t is the time, and τavg is the average decay time. The calculated τavg as a function of temperature is depicted in Figure 7b. Wu48 and Grinberg49 et al. discussed the spin−orbit interaction with the odd-parity vibration in the 3d3 system and proposed a complete model to describe the luminescent dynamics of Mn4+ at different temperatures. The temperature-dependent decay times in Figure 7b can be fitted by the following equation: τE =

) + 3exp(− kTΔ ) 2⎡ W 1 ℏω ⎤ Δ 1 + p′exp(− kT )⎦ + 3exp(− kT )} τ {( Δ ′ ) ⎣ (

ℏω

1 + exp − kT

S−O

T

(7)

where τE is the decay time of the 2E → 4A2 emission, WS−O is the spin−orbit coupling quantity, τT−1 is the radiative transition probability of the 4T2 → 4A2 emission, and ℏω is the effective energy of phonons. Δ′ is the energy difference between the 2E and 4T2 excited levels, and Δ is the energy difference between the minimum energy of the 2E and 4T2 states, which could be identified and calculated from the excitation and emission spectra (4A2 → 4T2 and 2E → 4A2) as shown in Figure 6b. Another process is a radiative transition τanti−1, which is associated with the anti-Stokes sideband and related with τT as

Figure 6. (a) Tanabe−Sugano diagram of a 3d3 system in an octahedral crystal field of Mn4+. (b) Schematic configuration coordinate diagram for Mn4+ ions in the Y2MgTiO6 double perovskite host.

the energy level of Mn4+ in an ideal octahedral crystal field. The crystal-field strength (10Dq) is determined by the peak energy of the 4A2 → 4T2 transition:45 4

4

Dq = E( A 2 − T2)/10

|W

(2)

parameter related with anti-Stokes sideband. Detail derivation process is shown in ref 49. In our case, Δ′ = 5846 cm−1 and Δ = 2400 cm−1, and the fitting parameters are ℏω = 225 cm−1, τT = 0.609 μs, WS−O = 87.8 cm−1, τanti = 0.69 ms, and p′ = 3.9.

On the basis of the energy difference between the A2 → T2 and 4A2 → 4T1 transitions in the excitation spectra, the Racah parameter B can be evaluated by the following equation:46 4

|2

the following equation: τanti−1 ≈ τT−1 (ΔS−′O)2 p′, where p′ is a free

4

E


Article

Inorganic Chemistry

Figure 7. (a) Temperature-dependent decay curves of the 698 nm emission in Y2MgTiO6:0.2%Mn4+ under the 355 nm pulsed YAG:Nd laser. (b) Calculated decay times of Y2MgTiO6:0.2%Mn4+ as a function of temperature, the red solid line represents the fitting curve. Figure 8. (a) Integrated emission intensity of Ianti‑S (620−673 nm), IS (673−760), and Itotal (620−760 nm) as a function of temperature. (b) Normalized temperature-dependent emission spectra of Y2MgTiO6:0.2%Mn4+ under the 355 nm excitation.

The results show that the value of ℏω in this work is close to that in Cr3+-doped LiTaO3 (200 cm−1),50 which belongs to the analogous perovskite structure. The calculated radiative lifetime (τT = 0.609 μs) is shorter than that in Rb2GeF6:Mn4+ (1.15 μs), indicating a little faster radiative transition of 4A2 → 4T2 in the Mn4+-doped oxide compounds.48 The spin−orbit coupling constant WS−O (87.8 cm−1) is induced by the perturbation approach, and the value is comparable to that in Cr3+-doped Garnets (105 cm−1).51 The obtained value of τanti (0.69 ms) represents the nature of spin- and parity-forbidden 2E → 4A2 transition with a millisecond scale. The value of parameter p′ (3.9) means that the contribution of the probability of the radiative transition induced by anti-Stokes phonon is ∼4 times larger than that caused by static crystal field for the 2E → 4A2 radiative transition probability.27,49 Application of Y2MgTiO6:Mn4+ in Optical Temperature Sensing. Generally, the transition-metal (3d3) ions of Mn4+ are strongly susceptible to the temperature because of the unshielded 3d electrons. As shown in Figure 8a, the total emission intensity (Itotal) of Y2MgTiO6:0.2%Mn4+ increases first and then decreases with increasing temperature. Such a trend in the sharp intensity change indicates that the Y2MgTiO6:Mn4+ phosphor is suitable for the optical temperature sensing. Figure

8b depicts normalized emission spectra as functions of temperature from 10 to 513 K. The strongest peak position of each curve is located at the region of Stokes sideband. The anti-Stokes sideband represents the continual increase in intensity in the normalized emission spectra. The thermal equilibrium usually exists in some trivalent rare-earth ions with adjacent energy gap in the range of 100−2000 cm−1.52 The intensity ratio of the anti-Stokes sideband and the Stokes sideband should obey the Boltzmann’s law, because the phonon-coupled sidebands can be considered as a couple of adjacent levels (∼600 cm−1). When the temperature increases, the anti-Stokes sideband (upper levels) can be thermally depopulated from the Stokes sideband (lower levels) to reach thermal equilibrium; as a result, the intensity of the anti-Stokes sideband elevates with increasing temperature. The relationship between the integrated intensity of the anti-Stokes sideband (Ianti‑S: 620−673 nm) and the Stokes sideband (IS: 673−760 nm) has the following formula: F


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Inorganic Chemistry

Figure 9. (a) FIR between Ianti‑S and IS as a function of temperature. (b) Calculated relative sensitivity SR at different temperatures.

FIR =

g σ2ω2 ⎛ ΔE ⎞ Ianti − S ⎟ + B ≈ 2 exp⎜ − ⎝ kT ⎠ IS g1σ1ω1

⎛ ΔE ⎞ ⎟ + B = A exp⎜ − ⎝ kT ⎠

that the Y2MgTiO6:0.2%Mn4+ has a wide and effective temperature sensing range from 10 to 513 K with the highest sensitivity at 153 K. When compared with the toxic Cs2WO2F4:Mn4+ (0.0021 K−1 at 167 K) of our previous work,27 the Y2MgTiO6:Mn4+ is a fluoride-free material and more environmentally friendly. Similar compound such as Prdoped La2MgTiO6 exhibits much better efficiency of the sensitivity at higher temperatures (0.072 K−1 at 450 K);53 however, we use Mn element instead of the costly Pr element as the activator, indicating the advantage in economy.

(8)

where Ianti‑S and IS are the luminescence intensities originated from upper and lower thermally coupled states, respectively. g, σ, and ω are the degeneracy, the emission cross section, and the angular frequency of luminescence transitions, respectively. A is the proportional parameter, k is the Boltzmann constant, ΔE is the energy gap between the two coupled states, and B is an offset parameter.54 As shown in Figure 9a, the fit result of the experimental data is well-consistent with the theory with the parameters A = 0.81, B = 0.02, and ΔE = 211 cm−1. The rate of change of the luminescence intensity ratio with temperature (dFIR/dT), named as temperature sensitivity (SR), is important to evaluate the suitability for thermometry and is given by SR =

⎛ −ΔE ⎞⎛ ΔE ⎞ dFIR ⎟⎜ ⎟ = A exp⎜ ⎝ kT ⎠⎝ kT 2 ⎠ dT

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CONCLUSION The Mn4+-doped Y2MgTiO6 phosphors were synthesized by a conventional high-temperature solid-state reaction method. Rietveld refinement method was performed to confirm the crystal structure. We deem that the credible crystal structure of Y2MgTiO6 is monoclinic double perovskite with space group P21/n. The excitation and emission spectra of Y2MgTiO6:Mn4+ illustrate wide absorption in the near-UV and blue light regions and exhibited deep red 2 E g → 4 A 2 emission. The Y2MgTiO6:Mn4+ phosphor with strong thermal quenching property has great potential as the optical thermometry with high thermal stability under UV excitation. The highest relative temperature sensitivity SR was calculated to be 0.001 42 K−1 at 153 K. Results exhibit that the commercial UV chips fabricated

(9)

The simulated results are displayed in Figure 9b. The sensitivity increases with increasing temperature and reaches the maximum at 153 K with 0.001 42 K−1; with further increase in temperature, the sensitivity decreases. The results indicate G


Article

Inorganic Chemistry Y2MgTiO6:Mn4+ is a potential noncontact optical thermometry in cryogenic industry.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82 51 629 5568. Fax: +82 51 629 5549. E-mail: [email protected]. ORCID

Hyo Jin Seo: 0000-0002-0490-8484 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (2017R1D1A1B03029432).

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Optical Thermometry Based on Vibration Sidebands in Y2MgTiO6

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