I've written about this many times before, but I still hear from many people who think that the way to get a higher score is by using big, difficult or 'original' words.
For example, a student commented recently that she had improved her English by learning the words ameliorate, juggernaut and katzenjammer. I can honestly say that I had never seen or heard the word 'katzenjammer' before. My dictionary says that it is an informal and old-fashioned word that was used in the US, which explains why I didn't know it. If you use a word like that in your IELTS essay, your writing will look very strange!
Please don't waste your time learning strange words. Instead, read through the writing lessons on this website, and try to copy the writing style that I use.