Optical Materials 99 (2020) 109572 Available online 6 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved. Effect of water vapor content during the solid state synthesis of manganese-doped magnesium fluoro-germanate phosphor on its chemistry and photoluminescent properties Amjad Ali a, b, c, * , Liudmyla M. Chepyga a, b , Laraib Sarfraz Khanzada a, d , Andres Osvet a , Christoph J. Brabec a , Miroslaw Batentschuk a a Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander-Universit€ at Erlangen-Nürnberg, Martensstraße 7, 91058, Erlangen, Germany b Energie Campus Nürnberg (EnCN), Fürther Str. 250, 90429, Nuremberg, Germany c Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore, Pakistan d Department of Metallurgical Engineering, NED University of Engineering and Technology, Karachi, Pakistan A R T I C L E INFO Keywords: Magnesium fluoro-germanate Solid state reaction Photoluminescence Thermographic phosphor Water vapor Effect of humidity ABSTRACT Several samples of magnesium fluoro-germanate doped with Mn 4þ were prepared by solid state synthesis with a different water vapor content in the synthesis atmosphere. A significant decrease in quantum yield and emission intensity was observed with the increase in water vapor content during synthesis. Although there was little effect on the emission spectra, the changes in excitation spectra indicate the dominance of magnesium germanate instead of magnesium fluoro-germanate at high water vapor content. X-ray diffraction (XRD) results show that the pure phase was only achieved with the least amount of water vapor. Refinement of XRD data estimates the quantities of different phases present. The effect on the grain size and morphology is also remarkable, as demonstrated by scanning electron microscopy imaging. The reason behind the changes is discussed in detail. The temperature dependence of the photoluminescence spectra was measured between room temperature and 500 � C where the luminescence is quenched to nearly 3% of its initial value. 1. Introduction Magnesium fluoro-germanate doped with Mn 4þ is an efficient deep- red phosphor having applications like color correction in mercury-vapor lamps, high color rendering index (CRI) light-emitting diodes (LEDs) and high-temperature thermography. In this host, Mn 4þ exhibits deep red emission peaking at 658 nm, retaining high efficiency even at high temperatures allowing its use for color correction in high-pressure mercury lamps where the temperature goes to 300 � C [1,2]. There are many recent investigations regarding chemistry and spectroscopic properties of this phosphor [3–6]. It has an excitation band centered at 420 nm, so it can be excited with a blue LED chip in a white light-emitting diode (WLED). Generally, WLEDs are made by combining an InGaN blue LED with yellow YAG : Ce 3þ phosphor. Although this approach yields a high correlated color temperature, the CRI is relatively low due to the weak emission in the red spectral range. To improve the CRI in traditional YAG : Ce 3þ WLEDs, a red emitting phosphor with suitable excitation spectrum is required. For more demanding lighting applications, a blend of red and green emitting phosphors can be applied. Choi et al. used a blend of red magnesium fluoro-germanate and green β  SiAlON : Eu 2þ on 420 nm blue LED chip to make a warm white LED [7]. Magnesium fluoro-germanate is one of the most investigated ther- mographic phosphors due to its detectable photoluminescence (PL) emission output even at high temperatures up to 800 K and its usefulness for both intensity-ratio and time-resolved methods [8]. Omrane et al. used this phosphor in multiple investigations for thermography including 2D temperature measurement of combustible and non-combustible surfaces and temperature measurement of decompos- ing construction materials by using the decay time method [9,10]. They also used the same phosphor for thermal imaging of single liquid drops by using the intensity ratio method [11]. Surface thermometry in the afterburner of aircraft engine was done with this phosphor by Seyfried et al. using the decay time method [12]. Tao Cai et al. used a three-gate * Corresponding author. Energie Campus Nürnberg (EnCN), Zimmer 16.1.17, Fürther Str. 250, 90429, Nuremberg, Germany. E-mail address: amjad.ali@fau.de (A. Ali). Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2019.109572 Received 31 August 2019; Received in revised form 19 November 2019; Accepted 22 November 2019