Studies on the Luminescence Properties of CaZrO3:Eu3+ Phosphors Prepared by the Solid State Reaction Method

Abstract CaZrO3:xEu3+ (x = 1.0, 2.0, 3.0, 4.0, and 5.0 mol%) phosphors were successfully prepared by a solid state reaction method. The crystal structure of sintered phosphors was hexagonal phase with space group of Pm-3m. The near ultra-violet (NUV) excitation, emission spectra of the CaZrO3:xEu3+ phosphors were composed of sharp line emission associated with the transitions from the excited states 5D0 to the ground state 7Fj (j = 0, 1, 2, 3, 4) of Eu3+. The results indicated that CaZrO3:xEu3+ might become an important orange-red phosphor candidate for use in white light emitting diodes (WLEDs) with near-UV LED chips. The mechanoluminescence (ML) intensity increases linearly with increasing impact velocity of the moving piston, suggesting that the sintered phosphors can also be useful as a stress sensor.


Introduction
Today lighting and display industries are focused upon developing efficient highintensity LED that produces white light. However, since white light is actually composed of many colors and LED produce monochromatic colors, this possesses a considerable challenge for LED technology [1,2]. Presently, engineers have developed three systems for producing white light with LED; mixing red, green and blue (RGB) LED, UV LED with RGB phosphor coatings and blue LED with yellow phosphor coatings [3,4]. For example, the commonly used red phosphor Y 2 O 2 S:Eu 3+ shows lower efficiency compared with those of blue and green phosphors and instability due to release of a sulfide gas [5]. So it is necessary to find new red or orange-red phosphors, which should have a stable host, exhibit strong absorption and emission under 400nm excitation. Recently, considerable efforts have been devoted to the research of new orange-red materials used for white LEDs [6]. Quite a lot of luminescent materials activated by rare earth ions have been invented.
Thus, it is very essential to search a new orange-red light that can be used effectively to compensate the orange-red emission deficiency of the LED output light. For general lighting, photoluminescent materials including oxides, silicates, aluminates, alumino-borates, aluminosilicates, nitrides, borates etc., play very important for the potential applications in ultraviolet devices [7][8][9][10][11][12]. Oxides with perovskite structures are important materials with tunable compositions. This class of materials has attracted tremendous attention for their functional properties, such as ferro-electricity, piezo-electricity, pyro-electricity, non-linear dielectric behavior, as well as multi-ferroic property with wide applications in electronic industries [13,14]. Among the perovskites calcium zirconate (CaZrO 3 ) is one of the material that has been extensively explored in the scientific community due its excellent electrical and thermo-M A N U S C R I P T

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Page | 3 mechanical properties. Because of its inherent character to exhibit proton conductivity even at high temperatures, it is an ideal candidate to be used in sensors [15]. In recent years, rare earth doped CaZrO 3 materials have been widely investigated due to their significance to fundamental research and their high potential for application in optical materials [16]. According to Longo et al., the displacement of Zr or Ca atoms in disordered perovskite CaZrO 3 may induce some vacancy defects at the axial and planar oxygen sites of the [ZrO 6 ] octahedral [17]. It is well known that the vacancy defects may play important roles as not only carriers traps but also luminescence centers.
The optical properties include the thermoluminescence (TL) as well as mechanoluminescence (ML) of the materials. TL is the discharge of stored energy by thermal stimulation in the form of light [4]. ML is a type of luminescence caused by mechanical stimuli such as grinding, cutting, collision, striking and friction [12]. Up to now, some phosphors with high ML, such as red phosphors (BaTiO 3 -CaTiO 3 :Pr), green phosphors (SrAl 2 O 4 :Eu), yellow phosphors (ZnS:Mn) have been developed. However, these phosphors have low water resistance and lack variety in color, which have limited the application of ML sensors. It is well known ZrO 2 has a low thermal conductivity, high melting point, high thermal and mechanical resistance.
It is used as an ideal medium for the fabrication of highly luminescent material due to its high refractive index, low phonon energy, high chemical and photochemical stability. ZrO 2 also plays an important role in the preparation of novel optical device materials [18].
In the present study, we have tested calcium zirconate (CaZrO 3 ) as a host lattice. Series of CaZrO 3 :xEu 3+ (x = 1.0, 2.0, 3.0. 4.0 and 5.0 mole%) phosphors were synthesized by the solid state reaction method. We report the structural characterization and optical properties of synthesized CaZrO 3 :xEu 3+ phosphors. The crystal structure and surface morphology were analyzed by X-ray diffractometer (XRD) and field emission scanning electron microscopy (FESEM). Luminescence properties were also investigated on the basis of photoluminescence (PL), CIE; color purity; decay, thermoluminescence (TL); TL spectra; mechanoluminescence (ML); ML decay and ML spectrum techniques. phosphors, then the powders were mixed and milled thoroughly for 3 hours using mortar and pestle. The chemical reaction used for stoichiometry calculation was:

Phosphors preparation
The ground samples were placed in an alumina crucible and subsequently fired at 1300 o C for 4 hours in an air. At last the nominal compounds were obtained after the cooling down of programmable furnace and products were finally grounded into powder for characterizing the phosphors.

Measurement techniques
The powder XRD pattern of the prepared CaZrO 3 :xEu 3+ phosphors have been obtained from the Bruker D8 advanced X-ray powder diffractometer using CuK (1.54060 Å) radiation  (19). All measurements were carried out at the room temperature.

XRD analysis
In order to determine the crystal structure of synthesized phosphors, powder XRD analysis has been carried out. Typical XRD patterns of CaZrO 3 and CaZrO 3 :xEu 3+ (x = 1.0, 2.0, 3.0, 4.0 and 5.0 mole%) phosphors with the standard XRD pattern was shown in Fig. 1  Standard data (JCPDS) file (JCPDS:20-0254) [20], indicating that the doping of Eu 3+ ions does not cause any significant change in the host structure. A comparison of the data with the standard JCPDS file reveals that the diffraction peaks of the CaZrO 3 :xEu 3+ phosphors match with those of the standard hexagonal phase with the space group of Pm-3m (221). The atomic parameters of CaZrO 3 phosphor were shown in Table 1.  should be expected to occupy the Ca 2+ sites, preferably, since the ionic radius of Eu 3+ (1.07 Å) is close to the Ca 2+ (1.12 Å) ions compared with the ionic radii of Zr 4+ (0.57 Å). Fig. 1  FESEM studies were carried out to obtain information about surface morphology, grain size and shape of the synthesized optimum CaZrO 3 :xEu 3+ (3.0%) phosphor. The morphologies of prepared CaZrO 3 :xEu 3+ (3.0%) phosphor was also observed by means of FESEM with different magnification in Fig. 1 (c). The micrographs demonstrate that the sample sizes are varying from a few microns to several tens of microns and form a large secondary particle. The surface of the discussed phosphor has shown irregular shape which means the distribution of the particle sizes was not homogeneous. From the FESEM image, it can be observed that the prepared phosphor consists of particles with different size distribution. FESEM examination showed that the particle shape and size of the solid state reaction depended significantly on the synthesis procedure. It is ascribed to that the solid state reaction used in this study requires a high temperature, which induces sintering and aggregation of particles, and it is an advantage for perfect crystal formation.

Photoluminescence (PL)
In order to facilitate the analysis of the optical properties of spectrum is composed of two major parts: (1) the broadband between 220 and 300 nm, the broad absorption band is called charge transfer (CT) state band due to the europium-oxygen interactions, which is caused by an electron transfer from an oxygen 2p orbital to an empty 4f shell of europium and the strongest excitation peak is at about 249 nm [21]. (2) A series of sharp lines between 300 to 500 nm, ascribed to the f-f transition of Eu 3+ . The strongest sharp peak is located at 395 nm corresponding to 7 F 0 → 5 L 6 transition of Eu 3+ ions. Other weak excitation peaks were located at 320, 363, 383, 417 and 466 nm are related to the intra-configurational 4f-4f transitions of Eu 3+ ions in the host lattices, which can be assigned to 7 7 F 0 → 5 D 3 and 7 F 0 → 5 D 2 transitions, respectively. The prepared CaZrO 3 :xEu 3+ phosphors can be excited by near UV (NUV) at about 395 nm effectively. So, it can match well with UV and NUV-LED, showing a great potential for practical applications [22].

Fig. 2 Excitation and emission spectra of CaZrO 3 :xEu 3+ phosphors with different Eu 3+ concentration
From the excitation and emission spectra of CaZrO 3 :Eu 3+ , the characteristics of this excitation spectrum showed some remarkable differences from that reported by Dubey et al. [23], which reported that the intensity of f-f absorption transition of Eu 3+ at 393 nm is much lower than that the CT absorption band (CTB absorption in CaZrO 3 :Eu 3+ is dominated). However, our experiment data indicated that the CTB absorption in CaZrO 3 :xEu 3+ is not dominated. As a result, it can match well with the radiation of NUV InGaN-based LED chip.  phosphors, prepared in our experiment, the strongest orange emission peak is located at 593 nm will be dominated. It can be presumed that Eu 3+ ions mainly occupy with inversion symmetric center in the host lattice [24].
To investigate the concentration dependent luminescent property of Eu 3+ ions doped be accompanied by an increase in the emitted light intensity, but it has been established that such behavior occurs up to a certain critical concentration. Above this critical concentration the luminescence intensity starts to decrease. This process is known as concentration quenching of the luminescence [25].
The concentration quenching is due to energy transfer from one activator (donor) to another until the energy sink (acceptor) in the lattice is reached. Hence, the energy transfer will strongly depend on the distance (R c ) between the Eu 3+ ions, which can be obtained using the following equation (1) [26].
Where X c is the critical concentration, Z is the number of cation sites in the CaZrO 3 unit cell [Z = 1 in CaZrO 3 ], and V is the volume of the unit cell (V = 64.92 (Å) 3 in this case). The critical concentration is estimated to be about x = 3.0 mole%, where the measured emission intensity begins to decrease. The critical distance (R c ) between the donor and acceptor can be calculated from the critical concentration, for which the nonradiative transfer rate equals the internal decay rate (radiative rate). Blasse [27,28] assumed that, for the critical concentration, the average shortest distance between the nearest activator ions is equal to the critical distance.
By taking the experimental and analytic values of V, Z and X c [64.92 (Å) 3 , 1, 3.0 mole%, respectively], the critical distance R c is estimated by Equation (1) is equal to 16.05 Å in this host.
The value of R c is greater than 5 Å for the rare earth ions indicating that the multipole-multipole interaction is dominant and is the major cause of concentration quenching of Eu 3+ in the phosphors.

CIE Chromaticity Coordinate
The chromaticity diagram is a tool to specify how the human eye will experience light with a given spectrum. The luminescence color of the samples were excited under 395 nm has been characterized by the CIE (Commission International de I'Eclairage) 1931 chromaticity diagram. The emission spectrum of the CaZrO 3 :Eu 3+ (3.0%) phosphor was converted to the CIE 1931 chromaticity using the photo-luminescent data and the interactive CIE software (CIE coordinates calculator) [29] diagram as shown in Fig. 3.

Decay
Where, I is phosphorescence intensity, A 1 , A 2 are constants, t is time, τ 1 and τ 2 are decay times (in millisecond) for the exponential components. Decay curves are successfully fitted by the equation (3) and the fitting curve result are shown in the inset of Fig. 4 with the standard error. The results indicated that the prepared CaZrO 3 :Eu 3+ (3.0%) phosphor shows a rapid decay and the subsequent slow decaying process [30].

Thermoluminescence (TL)
In order to study the trap states of the prepared CaZrO 3 :xEu 3+ (x = 1.0, 2.0, 3.0, 4.0 and 5.0 mole%) phosphors, TL glow curves were measured and shown in Fig. 5 (a). The synthesized phosphors were first irradiated for 5 min using 365 nm UV source, then the radiation source was removed and the irradiated samples were heated at a linear heating rate of 5 o C/s, from room temperatures to 250 o C. Initially, TL intensity increases with temperature, attains a peak value for a particular temperature and then it decreases with further increase in temperature. A single glow peak of CaZrO 3 :xEu 3+ phosphors were obtained at 113.31 o C. The single isolated peak due to the formation of only one type of luminescence center which is created due to the UV irradiation. It is suggested that the recombination center associated with the glow at the temperature interval arises from the presence of liberated pairs, which are probably the results from the thermal release of electron/holes from different kinds of traps and recombine at the color centers. It is also known that the doping of the rare earth ions increases the lattice defects which have existed already in the host. The position of the TL peaks keeps almost constant in the concentration range studied. It is observed that the intensity of this glow peak is found to increase with the increase of Eu 3+ concentration up to x = 3.0% and then decreases for higher concentration i.e.,

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Page | 14 for x = 3.0%. The TL intensity decrease due to concentration quenching of Eu 3+ ions. The TL signal steadily increased after incorporation of Eu 3+ ions, which are well known as efficient activators in many materials. In the present study it is observed that the glow curve shapes of europium doped samples are similar, indicating that there are interactions of intrinsic defects and doped impurities [33]. The different TL parameters calculations are listed in Table 2.  from TL glow curves [35]. TL parameters of prepared phosphors were calculated using the peak shape method. The relationship between the frequency factor's' and the activation energy 'E' is given by the equation (5) ) Where, k is Boltzmann constant, E is activation energy, b is order of kinetics, T m is temperature of peak position, and β is the heating rate. In the present work β = 5 o Cs -1 . Trap depth for second order kinetics is calculated using the equation (6)   Where, is the total half width intensity = + , is the half width at the low temperature side of the peak ( = T m -T 1 ); is the half width towards the fall-off side of the glow peak ( = T 2 -T m ), and T m is the peak temperature at the maximum. Chen provides a method which can identify the kinetics order for a model of one trap according to the shape of the TL band. The method involves the parameter g ( g = / ). The shape factor ( g ) is to differentiate between first and second order TL glow peak. ( g ) = 0.39 -0.42 for the first order kinetics, ( g ) = 0.42 -0.48 for the non-first order kinetics (mixed order) and ( g ) = 0.49 -0.52 for the second order kinetics [36]. In our case, for the CaZrO 3 :xEu 3+ phosphors; shape factor ( g ) is lying M A N U S C R I P T

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Page | 16 between 0.47 to 0.50, which indicates that it is a case of non-first order kinetics, approaching towards second order, responsible for deeper trap depth. Table 2 Activation Energy (E), Shape factor (µ µ µ µ g ) and Frequency Factor (s) for 5 minute

UV irradiated CaZrO 3 :xEu 3+ phosphors for different Eu 3+ concentration
The TL kinetic parameters of CaZrO 3 :Eu 3+ (3.0%) phosphor was also calculated by the peak shape method and details are given in Table 3. In our case, the value of shape factor ( g ) of CaZrO 3 :Eu 3+ (3.0%) phosphor was lies between 0.48 to 0.50, which indicates that it is a case of non-first order kinetics, approaching towards second order, responsible for deeper trap depth.
When the deep trap was created, the probability of re-trapping is high. It should also be noted that if the traps are too deep, it is not possible for UV excitation source to overcome the energy of a very deep trap at room temperature [37].

Mechanoluminecsnce (ML)
In the present ML studies, an impulsive deformation technique has been used [38]. When a moving piston (400gm load) was applied onto the phosphor, initially the ML intensity increases with time, attains a peak value and then decreases with time. Such a curve between the ML intensity and the deformation time of phosphors is known as the ML glow curve [39] Fig. 6 (a) shows that the comparative ML glows curve of CaZrO 3 :xEu 3+ phosphors for fixed height (h = 50 cm). The phosphor was fracture via dropping a load [moving piston] of particular mass and cylindrical shape on the CaZrO 3 :xEu 3+ phosphors. When the moving piston is dropped onto the prepared phosphors at 50 cm height, a great number of physical processes may occur within very short time intervals, which may excite or stimulate the process of photon emission and light is M A N U S C R I P T

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Page | 17 emitted. The photon emission time is nearly 2 ms, when prepared CaZrO 3 :xEu 3+ phosphors fractures. In these ML measurements, maximum ML intensity has been observed for CaZrO 3 :Eu 3+ (3.0%) phosphor. The prepared phosphor was fracture without any pre-irradiation such as X-ray, βrays, γ-rays, UV, etc.  The relationship between semi-log plot of ML intensity versus (t-t m ) for CaZrO 3 :Eu 3+ (3.0%) phosphor is shown in Fig. 7, and the lines were fitted using the following equation (7) with Origin 8.0 Curve fitting results show that the decay constant (τ) varies from 0.89 to 1.04 ms. The ML decay constant value is the maximum for the low impact velocities ( Table 4). The Decay rates of the exponentially decaying period of the ML curves did not change significantly with M A N U S C R I P T

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Page | 18 impact velocity. In order to further clarify of the ML decay mechanism in CaZrO 3 :Eu 3+ (3.0%) phosphor, more experimental and theoretical studies are needed.  When a mechanical stress, such as compress, friction, and striking, and so on, were applied onto the sintered CaZrO 3 :xEu 3+ phosphors, a local piezoelectric field can be produced.
Therefore, in such phosphors the ML excitation may be caused by the local piezoelectric field near the impurities and defects in the crystals [40]. With the increasing impact velocity, more compression of the sample takes place, and therefore, more area of the newly created surface takes place. Thus, the ML intensity will increase with increasing value of the impact velocity. It is to be noted that the stress near the tip of a moving crack is of the order of Y/100 ≈ 10 10 dynes/cm 2 = 10 9 Newton/m 2 (where Y is the Young's modulus of the materials). Thus, a fixed charge density will be produced on the newly created surfaces and the increase in the ML intensity will primarily be caused by the increase in M A N U S C R I P T

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Page | 19 the rate of newly created surface area with increasing impact velocity [42]. Moreover, the total ML intensity will also increase with impact velocity because the more compression of the sample will create more surfaces with increasing impact velocity. As the impact velocity increases, the impact pressure also increases, leading to the increase in the electric field at local region which causes the decrease in trap depth. Hence the probability of de-trapping increases. From Fig. 6 (b) (inset), it can be seen that with increasing impact velocity, ML intensity also increases linearly i.e., the ML intensity of CaZrO 3 :Eu 3+ (3.0%) phosphor is lineally proportional to the magnitude of the impact velocity, which suggests that this phosphor can be used as sensors to detect the stress of an object [43].      Table 3 Activation Energy (E), Shape factor (µ µ µ µ g ) and Frequency Factor (s) for CaZrO 3